N.Y. Comp. Codes R. & Regs. tit. 20, § 19-4.2
(b) The taxpayer's payroll factor, receipts factor and deposits factor are expressed as percentages and are determined as follows:
(3) The deposits factor is computed by dividing the average value of deposits maintained at branches of the taxpayer within New York State by the average value of deposits maintained at branches of the taxpayer within and without New York State. (See Subpart 19-7 of this Part.)
(c)
Tax Law, § 1454