Johnnie Sullivan was convicted following a jury trial on three counts of willful failure to file a tax return, in violation of 26 U .S.C. § 7203. He was sentenced on all three counts pursuant to the Sentencing Guidelines in effect at the time of sentencing, although the applicable tax offense guidelines had been amended after Mr. Sullivan had committed two of the three counts of willful failure to file. He argues on appeal that the application of the post-amendment guidelines to all three counts violates the ex post facto clause because it resulted in a higher guideline range than would the application of the pre-amendment guidelines to all three counts or the pre-amendment guidelines to the pre-amendment counts and the post-amendment guidelines to the post-amendment *1251count. Mr. Sullivan also argues the district court erred in using a 20%-of-gross-income figure to calculate the tax loss in this case. Finally, he argues in supplemental briefing filed just prior to oral argument of this case that the Supreme Court’s recent decision in Apprendi v. New Jersey, 530 U.S. 466, 120 S.Ct. 2348, 147 L.Ed.2d 435 (2000) invalidates his sentence because certain sentencing factors were not submitted to the jury. Because we conclude that the application of the post-amendment guidelines to pre-amendment conduct violates the ex post facto clause, but we find no error in the method used to calculate the relevant tax loss, we affirm in part and reverse and remand in part. We reject Mr. Sullivan’s Apprendi argument.
BACKGROUND
Mr. Sullivan was the owner and operator of Sullivan’s Rat Hole Drilling, a sole proprietorship that drilled holes for oil excavation. Mr. Sullivan stipulated that Sullivan’s Rat Hole Drilling earned gross receipts of $606,000 in 1991, $564,765 in 1992, and $517,253 in 1993. He failed to file tax returns for the years 1991, 1992 and 1993. The jury convicted him on all three counts of willful failure to pay.
The district court used the November 1, 1998, edition of the sentencing guidelines to calculate Mr. Sullivan’s sentence. Pursuant to U.S.S.G. § 3D1.2(d), the three counts were grouped and the initial base offense level was calculated by reference to the total aggregate amount of loss.1 The court determined that Mr. Sullivan’s total gross income for the three year period was $1,688,017 and that, pursuant to U.S.S.G. §§ 2Tl.l(a)(2) and 2Tl.l(c)(2), the tax loss was 20% of that figure, or $337,603. This gave Mr. Sullivan a base offense level of 17. After a two-level enhancement for sophisticated concealment, pursuant to § 2Tl.l(b)(2), Mr. Sullivan’s total offense level was 19. This resulted in a sentencing range of thirty to thirty-seven months. The court sentenced him at the low end of the range — thirty months— amounting to ten months on each count of conviction, to run consecutively. Mr. Sullivan did not object at the time of sentencing to the court’s use of the 1998 sentencing guidelines.
DISCUSSION
Mr. Sullivan concedes that, because his trial counsel did not object to the application of the November 1, 1998, guidelines to his sentence, we review his sentence under the guidelines only for plain error. See United States v. Gilkey, 118 F.3d 702, 704 (10th Cir.1997); Fed. R.Crim.P. 52(b). The plain error standard applies when an ex post facto claim is not raised at sentencing. See United States v. Massey, 48 F.3d 1560, 1568 (10th Cir.1995). “To constitute plain error, the error must have been both ‘obvious and substantial .... An error is substantial if it seriously affect[s] the fairness, integrity, or public reputation of judicial proceedings.’ ” United States v. Gerber, 24 F.3d 93, 95 (10th Cir.1994) (quoting United States v. Brown, 996 F.2d 1049, 1053 (10th Cir.1993)). An ex post facto application of a sentencing guideline which disadvantages the defendant can amount to plain error. United States v. Orr, 68 F.3d 1247, 1252 (10th Cir.1995). “We review de novo questions of law regarding application of the sentencing guidelines, and review for clear error the district court’s factual findings.” United States v. Spencer, 178 F.3d 1365, 1367 (10th Cir.1999).
I. Ex Post Facto Argument
The sentencing guidelines applicable to tax offenses were amended effective No*1252vember 1, 1993. The amendment increased the base offense level for failure to file convictions. Mr. Sullivan was convicted of failing to file tax returns on April 15, 1992, April 15, 1993, and April 15, 1994. Thus, two of the counts related to conduct occurring before the guidelines amendment and one count related to conduct occurring after the amendment. His total offense level under the pre-amendment guidelines was 15. Under the post-amendment guidelines, it was 19.
The Sentencing Guidelines contain the “one-book rule”: “The Guidelines Manual in effect on a particular date shall be applied in its entirety. The court shall not apply, for example, one guideline section from one edition of the Guidelines Manual and another guideline section from a different edition of the Guidelines Manual.” U.S.S.G. § lBl.ll(b)(2), p.s. We, like virtually every other circuit, have approved use of the one-book rule. See United States v. Nelson, 36 F.3d 1001, 1004 (10th Cir.1994). The Guidelines also state that, in general, a sentencing court must apply the guidelines in effect at the time of sentencing, see U.S.S.G. § 1B1.11(a), p.s., unless such application would violate the ex post facto clause. See U.S.S.G. § lBl.U(b)(l), p.s.; see also United States v. Svacina, 137 F.3d 1179, 1186 (10th Cir.1998). “The Ex Post Facto Clause is violated if the court applies a guideline to an event occurring before its enactment, and the application of that guideline disadvantages the defendant ‘by altering the definition of criminal conduct or increasing the punishment for the crime.’ ” Id. (quoting Lynce v. Mathis, 519 U.S. 433, 441, 117 S.Ct. 891, 137 L.Ed.2d 63 (1997)).
Additionally, in 1993, the Sentencing Commission issued a policy statement making explicit that the one-book rule applies to situations involving multiple counts: “[i]f the defendant is convicted of two offenses, the first committed before, and the second after, a revised edition of the Guidelines Manual became effective, the revised edition of the Guidelines Manual is to be applied to both offenses.” U.S.S.G. § lBl.ll(b)(3), p.s. The commentary to that provision states that the revised edition is to be applied “even if the revised edition results in an increased penalty for the first offense.” U.S.S.G. § lBl.ll(b)(3), comment. (backg’d.).2 That commentary also explains why the Commission perceives no ex post facto problem with § lBl.ll(b)(3):
Because the defendant completed the second offense after the amendment to the guidelines took effect, the ex post facto clause does not prevent determining the sentence for that count based on the amended guidelines. For example, if a defendant pleads guilty to a single count of embezzlement that occurred after the most recent edition of the Guidelines Manual became effective, the guideline range applicable in sentencing will encompass any relevant conduct (e.g., related embezzlement offenses that may have occurred prior to the effective date of the guideline amendments) for the offense of conviction. The same would be true for a defendant convicted of two counts of embezzlement, one committed before the amendments were enacted, and the second after. In this example, the ex post facto clause would not bar application of the amended guideline to the first conviction; a contrary conclusion would mean that such defendant was subject to a lower guideline range than if convicted only of the second offense.
Id. Mr. Sullivan argues that the application of the November 1, 1998, guidelines to all three counts, consistent with § 1B1.11(b)(3), violates the ex post facto clause. Both parties agree that application of the amended guidelines to Mr. Sul*1253livan’s pre-amendment conduct (the first two counts of conviction), or to all three counts, disadvantaged Mr. Sullivan by subjecting him to a higher sentencing range.
As both parties acknowledge, courts which have addressed the question whether sentences consistent with § lBl.ll(b)(3) violate the ex post facto clause are divided. Several have held that, consistent with the Sentencing Commission’s view, sentences imposed in accordance with § lBl.ll(b)(3) do not violate the ex post facto clause. See United States v. Lewis, 235 F.3d 215, 217 (4th Cir.2000) (“We conclude that the guideline provision of which [defendant] complains, U.S.S.G. § lBl.ll(b)(3), does not violate the Ex Post Facto Clause.”); United States v. Vivit, 214 F.3d 908, 919 (7th Cir.) (holding that the guidelines “provide notice to criminals that engaging in ongoing fraudulent behavior involving the same type of harm risks grouping of convictions, which because of the one-book rule, will all be sentenced according to the Guidelines in effect when the latest conduct occurred.”), cert. denied, — U.S. -, 121 S.Ct. 388, 148 L.Ed.2d 299 (2000); United States v. Kimler, 167 F.3d 889, 893 (5th Cir.1999) (same); United States v. Bailey, 123 F.3d 1381, 1404-05 (11th Cir.1997) (same); United States v. Cooper, 35 F.3d 1248 (8th Cir.) (same), vacated, 514 U.S. 1094, 115 S.Ct. 1820, 131 L.Ed.2d 742 (1995), reinstated, 68 F.3d 761 (8th Cir. 1995), cert. denied, 517 U.S. 1158, 116 S.Ct. 1548, 134 L.Ed.2d 650 (1996); United States v. Regan, 989 F.2d 44, 48 (1st Cir.1993) (same); United States v. Tucker, 982 F.Supp. 1309 (N.D.Ill.1997) (same).
Two circuits, however, and one district court, have held that such sentences violate the ex post facto clause. In United States v. Bertoli, 40 F.3d 1384 (3d Cir.1994), the defendant was convicted on two counts of obstruction of justice, one occurring in 1990 and one in 1992. The district court grouped the counts for sentencing purposes and applied a 1991 amendment which increased the defendant’s sentence. It held that, although one count involved conduct occurring in 1990 (prior to the amendment of the guideline), because that count was grouped with conduct occurring in 1992 (post-amendment), application of the amended guideline did not violate the ex post facto clause. The Third Circuit disagreed: “The fact that various counts of an indictment are grouped cannot override ex post facto concerns.” Bertoli, 40 F.3d at 1404. The court did not, however, reject application of the one-book rule. Rather, the court held that, when ex post facto concerns arise, the sentencing court can apply the one-book rule without violating the ex post facto clause by applying the pre-amendment guidelines to all counts.
The Ninth Circuit reached a similar conclusion, though with a slightly different solution to the problem, in United States v. Ortland, 109 F.3d 539 (9th Cir.1997). In Ortland, the defendant was convicted on five counts of mail fraud, four of which involved conduct occurring before a November 1, 1989, amendment changing the way loss is calculated in a fraud case. The Ninth Circuit held that application of § lBl.ll(b)(3) to sentence the defendant on all five counts under the amended guideline violated the ex post facto clause. But rather than applying the pre-amendment guidelines to all counts, the Ninth Circuit held the district court must apply the pre-amendment guidelines to the counts involving conduct occurring prior to the amendment, and the post-amendment guidelines to conduct occurring after the amendment. As the Ortland court explained:
Application of the policy statement [§ lBl.l.ll(b)(3) ] in this case would violate the Constitution; its application would cause [defendant’s] sentence on earlier, completed counts to be increased by a later Guideline. Moreover, the Commission’s explanation is not entirely logical. The harm caused by the earlier offenses can be counted in sentencing the later one. That does not mean that the punishment for the earlier offenses themselves can be increased, simply because the punishment for the later one can be. In fact, were the later count to *1254fall at some time after sentencing, all that would remain would be the earlier sentences, which would be too long.
Id. at 547. Accord, United States v. Johnson, Nos. 97-CR-206, 98-CR-160, 1999 WL 395381 (June 4, 1999), aff'd, United States v. Johnson, 221 F.3d 83 (2d Cir.2000).3
After careful consideration, we elect to follow the reasoning of the Third and Ninth Circuits and conclude that the application of guidelines policy statement § lBl.ll(b)(3) to the first two of Mr. Sullivan’s willful failure to file tax counts violates the ex post facto clause and is plain error requiring us to vacate his sentence.
Mr. Sullivan was charged for and convicted of three separate and discreet tax offenses. Although the three offenses were indisputably related, they nonetheless did not constitute a continuing offense. Thus, it is irrelevant that we have held that “when a crime involves a continuing violation, application of a law enacted after the crime begins does not implicate the ex post facto clause.” United States v. Boyd, 149 F.3d 1062, 1068 (10th Cir.1998).
As indicated above, the central concern of the ex post facto clause is fair notice to a defendant that the punishment for a crime has been increased from what it was when the crime was committed: “[o]ne function of the Ex Post Facto Clause is to bar enactments which, by retroactive operation, increase the punishment for a crime after its commission.” Gamer v. Jones, 529 U.S. 244, 120 S.Ct. 1362, 1367, 146 L.Ed.2d 236 (2000). Mr. Sullivan was on notice when he committed his first two discrete tax offenses that the Sentencing Guidelines then in effect would determine his sentence. He was simply not on notice that his sentence for those two offenses would be calculated under an amended guideline enacted after he completed those crimes. By contrast, it is fair to conclude that he was on notice when he committed his third discrete tax offense that the guideline applicable to such an offense had been amended and the punishment increased. We agree with the Third Circuit that the grouping rules “cannot override ex post facto concerns.” Bertoli, 40 F.3d at 1404. While Congress and/or the Commission can provide for an enhancement of a sentence because of prior violations, that is not what Mr. Sullivan experienced here. Rather, the punishment for his first two tax violations was actually retroactively increased. We therefore conclude that the application of § lBl.ll(b)(3) to sentence Mr. Sullivan for all counts under the amended guidelines violates the ex post facto clause.
However, we prefer the resolution endorsed by the Ninth Circuit in Orland to that of the Third Circuit in Bertoli. We therefore remand this case for resentencing, and we direct the district court to apply the pre-amendment guideline to Mr. Sullivan’s two pre-amendment failure to file convictions and the post-amendment guideline to Mr. Sullivan’s post-amendment conviction. Although such a resolution requires that the one-book rule not apply in a case such as this, it more narrowly addresses the ex post facto problem.4
*1255II. Calculation of Loss
Mr. Sullivan also argues the district court erred in the way it calculated the tax loss caused by his failure to file. While we have already determined that this case must be remanded for resentencing, we address this issue because, presumably, the issue of loss calculation will arise again on remand.
Prior to the 1993 guideline amendments at issue in this case, U.S.S.G. § 2T1.2(a) (1992) governed the calculation of tax loss for the willful failure to file a tax return. It defined tax loss as “the total amount of tax that the taxpayer owed and did not pay, but, in the event of a failure to file in any year, not less than 10 percent of the amount by which the taxpayer’s gross income for that year exceeded $20,000.” U.S.S.G. § 2T1.2(a)(2) (1992). The commentary explained as follows: “An alternative measure of the tax loss, 10 percent of gross income in excess of $20,000, has been provided because of the potential difficulty of determining the amount of tax the taxpayer owed. It is expected that this alternative measure generally will understate the amount of tax owed.” U.S.S.G. § 2T1.2, comment, (backg’d.).
Following the 1993 amendments, U.S.S.G. § 2Tl.l(e)(2) governs the calculation of tax loss in such -cases. It provides in part:
(2) If the offense involved failure to file a tax return, the tax loss is the amount of the tax that the taxpayer owed and did not pay.
Note: If the offense involved failure to file a tax return, the tax loss shall be treated as equal to 20% of the gross income ... less any tax withheld or otherwise paid, unless a more accurate determination of the tax loss can be made.
The commentary to § 2T1.1 states that the “presumption” of § 2T1.1(c)(2) is “to be used unless the government or defense provides sufficient information for a more accurate assessment of the tax loss.” U.S.S.G. § 2T1.1, comment. (n.l). That commentary further recognizes that there may be situations where the tax loss “may not be reasonably ascertainable.” Id. “[AJlthough the government bears the burden at sentencing of proving the amount of tax loss flowing from the defendant’s illegal acts, neither the government nor the court has an obligation to calculate the tax loss with certainty or precision.” Spencer, 178 F.3d at 1368 (citations omitted).
At trial, the government and Mr. Sullivan entered into a stipulation that Sullivan’s Rat Hole Drilling had the following gross business receipts: $606,000 in 1991, $564,765 in 1992 and $517,253 in 1993. They also stipulated that the business paid the following expenses: $424,200 in 1991, $395,336 in 1992 and $362,007 in 1993. See R. Vol. 1 at Doc. 47. The stipulation provided that it “is for purposes of the criminal trial only, and does not bind either party in any post trial matters or any civil proceeding.” Id.
In the presentence report, the probation officer determined that the tax loss was $337,603: “[p]ursuant to U.S.S.G. § 2Tl.l(c)(2), the tax loss for the years 1991 through 1993 is 20% of [Mr. Sullivan's] $1,688,017.20 gross income or $337,603.44.” PSR at 7, R. Vol. 8. Mr. Sullivan objected to the PSR’s use of the 20% figure because it “failed to include allowable trade and business deductions.” Addendum to PSR (Objections) at i, R. Vol. 8. The probation officer responded that the PSR’s tax loss figure should be used because “the defendant has failed to demonstrate that the tax loss information *1256in the Presentence Report is inaccurate.” Id. at ii.
At sentencing, the government introduced as an exhibit the gross income figure to which the parties stipulated at trial. Mr. Sullivan objected that it “shows only gross income.” Tr. of Sentencing at 32, R. Vol. 7. The government’s response was as follows:
As far as the deductions, again I will state that the guidelines say very clearly that you take 20 percent of the gross income unless you’ve got better figures. The defendant destroyed his records. The defendant would not provide information as far as his deductions. And although the Government has records of what expenses were and those records were turned over to the defense, there’s been no attempt in the last four months to sit down and — everything was intermingled — personal expenses, business expenses — as to what was paid for how, what was paid for in cash. There’s simply no way of determining that with any degree of accuracy.
And therefore, the Court is forced to fall back on the flat 20 percent figure, which is probably not a perfect number but it is the best we can do in this case and that’s because of the defendant’s lack of cooperation.
Id. at 40. After hearing argument from both sides on the issue, the court held it would follow the presumptive 20% figure, using the stipulated gross income figures: “[A]t this point I don’t believe the Court has before it information that assures the Court that a more accurate determination of the tax loss can be made ... in this matter. Certainly I don’t have the materials before me that would lead to that nor is there any present ability to arrive at that kind of calculation.” Id. at 50.
We affirm the district court’s use of the presumptive 20% figure. The guidelines state that the tax loss in a failure to file offense is 20% of the gross income “unless a more accurate determination of the tax loss can be made.” U.S.S.G. § 2Tl.l(c)(2) (emphasis added).5 After listening to arguments and reviewing exhibits and proposed exhibits on the matter, the court found that it could not determine a more accurate tax loss figure. Bearing in mind that we review the court’s factual findings for clear error, and that we must give “ ‘due deference’ to the district court’s application of the guidelines to the facts,” Spencer, 178 F.3d at 1367 (quoting United States v. Henry, 164 F.3d 1304, 1310 (10th Cir.), cert. denied, 527 U.S. 1029, 119 S.Ct. 2381, 144 L.Ed.2d 784 (1999)), we perceive no clear error in the district court’s computation of tax loss. Of course, on remand, the district court will have to calculate tax loss under both the pre- and post-amendment guidelines. We simply hold that, in the absence of additional evidence from which the court may determine the relevant tax loss with more accuracy, we perceive no error in its use of the presumptive figures.
III. Effect of Apprendi
In Apprendi, the Supreme Court held that “[o]ther than the fact of a prior conviction, any fact that increases the penalty for a crime beyond the prescribed statutory maximum must be submitted to a jury, and proved beyond a reasonable doubt.” Apprendi, 120 S.Ct. at 2362-63. We have not yet ruled on whether Apprendi extends to sentencing guideline factors. Mr. Sullivan concedes that the law in this circuit currently states that sentencing guideline factors need not be alleged in the indictment or found by the jury. See United States v. Frederick, 897 F.2d 490, 491-93 (10th Cir.1990). Other circuits have held that Apprendi does not apply to sentencing factors that increase a defendant’s guideline range but do not increase the statutory maximum. See Lewis, 235 F.3d at 217; United States v. Hernandez-Guardado, 228 F.3d 1017 (9th Cir. *12572000); United States v. Corrado, 227 F.3d 528 (6th Cir.2000); United States v. Meshack, 225 F.3d 556 (5th Cir.2000), petition for cert. filed, (U.S. Nov. 26, 2000) (No. 00-7246); United States v. Aguayo-Delgado, 220 F.3d 926 (8th Cir.2000), cert. denied; — U.S. -, 121 S.Ct. 600, 148 L.Ed.2d 513 (2000). We agree with those circuits. Since the tax loss calculation and two-level increase for sophisticated concealment did not increase Mr. Sullivan’s sentence beyond the statutory maximum of 12 months for each count of failure to file, his sentence does not run afoul of the rule in Apprendi.
CONCLUSION
For the foregoing reasons, we AFFIRM in part and REVERSE and REMAND for further proceedings consistent with this opinion.
. Section 3D 1.1 (a) provides for the grouping of "Closely Related Counts” as specified in § 3D 1.2. Section 3D 1.2, in turn, provides that “[a]ll counts involving substantially the same harm shall be grouped together into a single Group.” Among such counts "involving substantially the same harm” are counts where the "offense level is determined largely on the basis of the total amount of harm or loss.” Offenses encompassed by § 2T1.1, willful failure to file tax returns, are specifically included as counts which must be grouped.
. "[C]ommentary in the Guidelines Manual that interprets or explains a guideline is authoritative unless it violates the Constitution or a federal statute, or is inconsistent with, or a plainly erroneous reading of, that guideline.” Stinson v. United States, 508 U.S. 36, 38, 113 S.Ct. 1913, 123 L.Ed.2d 598 (1993).
. Although the Second Circuit affirmed the sentence imposed in Johnson, it declined to entertain the government's cross-appeal and the Second Circuit did not explicitly address the district court’s ex post facto analysis. The Second Circuit has, in another case, recognized "the possibility that the one-book rule might not be fully applicable to sentences based on multiple counts.” United States v. Santopietro, 166 F.3d 88, 96 (2d Cir.1999).. The Santopietro court examined the disagreement between circuits on the validity of § IB 1.11(b)(3), but declined to address the issue in the case before it.
. Our decision in United States v. Nelson, 36 F.3d 1001, 1004 (10th Cir.1994), in which we "join[ed] other circuits in adopting the ‘One Book’ rule,” does not compel a different result. In Nelson, the defendant pled guilty to a conspiracy and was sentenced under the 1992 guidelines which were in effect at the lime of sentencing. He argued the application of those guidelines, rather than the 1988 guidelines in effect when the offense conduct was committed, violated the ex post facto clause. The defendant also argued we should, in effect, apply both the 1992 and 1988 guidelines to calculate his sentence. We rejected such *1255dual application, noting that he "may not select piecemeal from the 1988 and 1992 Guidelines to come up with the most advantageous combination of provisions from the two books, but must instead be sentenced under one Guidelines Manual.” Id. But we also explicitly found no ex post facto problem: “the court's use of the 1992 Guidelines rather than the 1988 Guidelines did not disadvantage [defendant] because he received the same punishment under either version of the Guidelines.” Id. Thus, we have implicitly acknowledged that the one-book rule may not "trump” the ex post facto clause.
. The pre-amendment guidelines acknowledge the necessity of providing a presumptive figure because of the "potential difficulty” in determining the amount of tax owed. Moreover, they expect that the use of such a figure will be in the taxpayer’s favor.