California Code, Revenue and Taxation Code - RTC § 18004

(a) Credit shall be allowed only for such proportion of the taxes paid to the other state as the income taxable under this part and also subject to tax in the other state bears to the entire income upon which the taxes paid to the other state are imposed.

(b) The credit shall not exceed such proportion of the tax payable under this part as the income subject to tax in the other state and also taxable under this part bears to the entire income taxable under this part.