Tax Law, § 637(d)
The Tax Commission may, on application, authorize the use of such other methods of determining a nonresident partner's portion of partnership items derived from or connected with New York State sources, and the modifications related thereto, as may be appropriate and equitable on such terms and conditions as it may require.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 137.4