Tax Law, § 621(b)
The amount of the credit allowable under this Part may not reduce the New York State personal income tax otherwise due from the resident beneficiary to an amount less than the amount which would have been due if the accumulation distribution and the New York fiduciary adjustment applicable thereto were excluded from the beneficiary's New York adjusted gross income.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 121.2