N.Y. Comp. Codes R. & Regs. tit. 20 § 132.25
Sections 132.15 through 132.24 of this Part are designed to apportion and allocate to New York State, in a fair and equitable manner, a nonresident's items of income, gain, loss and deduction attributable to a business, trade, profession or occupation carried on partly within and partly without New York State. Where the methods provided under those sections do not so allocate and apportion those items, the department may require a taxpayer to apportion and allocate those items under such method as it prescribes, as long as the prescribed method results in a fair and equitable apportionment and allocation. A nonresident individual may submit an alternative method of apportionment and allocation with respect to items of income, gain, loss and deduction attributable to a business, trade, profession or occupation carried on partly within and partly without New York State. The proposed method must be fully explained in the taxpayer's New York State nonresident personal income tax return. If the method proposed by the taxpayer is approved by the department, it may be used in lieu of the applicable method under sections 132.15 through 132.24 of this Part.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 132.25