Tax Law, § 641(c)
The specific deduction of a nonresident individual, or married individuals filing a joint return, is an amount which bears the same ratio to $5,000 (or $2,500, in the case of a married individual filing a separate return) as the New York items of tax preference (determined as explained in section 141.1[b][1] of this Part) bear to the total New York items of tax preference (determined as explained in section 122.1[b] [1] of this Title), computed as if such nonresident taxpayer were a resident. The amount of the specific deduction allowable under this paragraph may not exceed $5,000 in the case of nonresident individuals, or married individuals filing a joint return, or $2,500 in the case of a married individual filing a separate return.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 141.2