If the amount determined under subdivision (i) of this subparagraph does not exceed the amount determined under subdivision (ii) of this subparagraph, no credit is allowable. If the amount determined under subdivision (i) of this subparagraph exceeds the amount determined under subdivision (ii) of this subparagraph, the credit allowable is the lesser of the amount of such excess or the amount of taxes deemed distributed to the beneficiary for all preceding taxable years to the extent that such taxes are not used in § 1.668(b)-1A(b)(2) or § 1.669(b)-1A(b)(2) in determining the beneficiary's partial tax under section 668(a)(2) or 668(a)(3). The application of this subparagraph may be illustrated by the following example:
Example. An accumulation distribution made in 1975 is deemed distribution in 1973 and 1974, years in which the beneficiary was in being. The taxes deemed distributed in such years are $4,000 and $2,000, respectively, totaling $6,000. The amounts of tax computed under § 1.668(b)-1A(b)(1) attributable to the amounts thrown back are $3,000 and $2,000, respectively, totaling $5,000. The credit allowable under this subparagraph is therefore $1,000 ($6,000 less $5,000).
If the amount determined under subdivision (i) of this subparagraph does not exceed the amount determined under subdivision (ii) of this subparagraph, no credit is allowable. If the amount determined under subdivision (i) of this subparagraph exceeds the amount determined under subdivision (ii) of this subparagraph,
Example. An accumulation distribution that consists only of undistributed net income is made in 1975. The taxes deemed distributed in the preceding years under §§ 1.666(b)-1A and 1.666(c)-1A are $15,000. The amount determined under § 1.668(b)-1A(c)(1)(v) is $12,000. The beneficiary was in being on the last day of all but one preceding taxable year in which the accumulation distribution was deemed made, and the taxes deemed distributed in those years was $10,000. Therefore, the excess of the subdivision (i) amount over the subdivision (ii) amount is $3,000, and is multiplied by 10,000/15,000, resulting in an answer of $2,000, which is the credit allowable when computed under the short-cut method.
26 C.F.R. §1.667(b)-1A