If the commissions for sales made or other compensation for services performed by a nonresident traveling salesman, agent or other employee depend directly upon the volume of business transacted by him, his items of income, gain, loss and deduction (other than deductions entering into the New York itemized deduction) derived from or connected with New York State sources include that proportion of the net amount of such items attributable to such business which the volume of business transacted by him within New York State bears to the total volume of business transacted by him within and without New York State.
N.Y. Comp. Codes R. & Regs. Tit. 20 § 132.17