California Code, Revenue and Taxation Code - RTC § 17304

In the case of a nonresident or part-year resident, itemized deductions allowed as a deduction for the taxable year under Section 63 of the Internal Revenue Code, as modified by Section 17073, or the standard deduction (as provided in Section 17073.5), shall be allowed in computing “taxable income of a nonresident or part-year resident” in the ratio (not to exceed 1.00) that California adjusted gross income (as defined in Section 17301.3) bears to total adjusted gross income (as defined in Section 17301.4).