Cal. Rev. & Tax. Code § 17301

For purposes of this part, in the case of a nonresident or part-year resident, the proper apportionment and allocation of the deductions in computing “taxable income of a nonresident or part-year resident” computed under paragraph (1) of subdivision (i) of Section 17041 with respect to sources of income within and without the state shall be determined under rules and regulations prescribed by the Franchise Tax Board.