dissenting in part.
I respectfully disagree with the court's ultimate resolution of Tahni's October 16, 2006, receipt of $18,817.50 in post-judgment interest. The court acknowledges Douglas's argument that the interest is income that should be included in the Rule 90.3(a) caleu-lation of Tahni's 2006 income, and further acknowledges that the receipt of interest generally is income for Rule 90.3 purposes. The court appropriately expresses a legitimate concern that including the entire amount of the interest "would paint a distorted picture of Tahni's 2006 income." But in attempting to allay its concern, the court goes astray by creating a new rule of law that abridges the trial court's discretion to calculate an appropriate child support award.1
Although true that the question of "whether an item qualifies as income for the purposes of Rule 90.3 is a question of law,"2 calculating income for future child support payments is a matter within the trial court's discretion. The commentary to Rule 90.3 observes:
Child support is calculated as a certain percentage of the income which will be earned when the support is to be paid. This determination will necessarily be somewhat speculative because the relevant *1214income figure is expected future income. The court must examine all available evidence to make the best possible calculation.[3]
In my view the court should allow the superi- or court the opportunity on remand to consider all evidence of Tahni's 2006 and future income, including but not limited to Douglas's one-time payment of long-acerued interest, and to exercise its discretion in fashioning an appropriate child support award for 2006 onward.
Tahni has asked the superior court to lower her child support obligation, and a modification of a child support obligation is within the discretion of the superior court. The superior court certainly is capable of: (1) recognizing the income distortion created by Douglas's one-time payment of the entire eleven and one-half years of post-judgment interest; (2) exercising its authority and discretion to apply income-averaging to this non-recurring source of income in its calculations, if appropriate;4 and (8) recognizing and exercising its discretion to vary the child support award under the Rule 90.3(a) formula to avoid manifest injustice, if good cause to do so exists.5 After examining all evidence of Tahni's income for 2006 and beyond, the superior court may determine that some, all, or none of the interest payment should be taken into account in the Rule 90.3(a) calculations or that a variation from those calculations is warranted under Rule 90.3(c).
Instead the court fashions a new rule on what constitutes income for purposes of Rule 90.3. The court holds as a matter of law that when a non-recurring payment of acerued interest is received in a given year, only the amount of interest that actually accrued in that year is income for Rule 90.8(a) calculations. The rule is inappropriate because it cannot reasonably fit all facts and circumstances and may encourage manipulation of the timing for receipts of income. If Douglas had paid Tahni on January 1, 2006, as a matter of law only one day's interest would be income for Rule 90.3(a). In other situations the party receiving interest income might cause it to be received January 1 to ensure that virtually all of it is not treated as Rule 90.3 income, which may not be fair to either the custodial parent or the affected children. The rule is also unnecessary because, at least until today, the superior court had the authority to evaluate unusual receipts of income and take them into account along with all other relevant facts and circumstances, and then had broad discretion to determine an appropriate child support award.6
If the court is concerned that Douglas unfairly attempted to skew Tahni's income in his favor after she moved for modification of her support obligation, the proper response is not to tic the lower courts' hands in all cases with a new rule of law fashioned for this particular case. Instead, it is to allow the superior court to exercise its sound discretion by making the best possible caleulation of Tahni's income going forward in light of all available evidence.
. See Keturi v. Keturi, 84 P.3d 408, 412 (Alaska 2004) ("We will not overturn a child support award unless the trial court abused its discretion in calculating the award.").
. Robinson v. Robinson, 961 P.2d 1000, 1002 (Alaska 1998).
. Alaska R. Civ. P. 90.3, Commentary IILE, 232 (2008). "The commentary to Civil Rule 90.3 has not been officially adopted, but it can provide useful guidance in applying the rule." Miller v. Clough, 165 P.3d 594, 600 n. 10 (Alaska 2007) (citing Caldwell v. State, 105 P.3d 570, 573 n. 6 (Alaska 2005)).
. See Keturi 84 P.3d at 413 (noting our holdings that income-averaging may be used to deal with erratic income).
. See Alaska R. Civ. P. 90.3(c)(1).
. Keturi, 84 P.3d at 412.