Cal. Rev. & Tax. Code § 24370

There shall also be allowed as a deduction, under Chapter 2 of this part, in the case of a mutual savings bank, the entire amount of interest paid to depositors possessing no proprietary interest in the institution or in its surplus, and interest on their deposits to members possessing a proprietary interest in the institution or in its surplus at a rate determined by the Commissioner of Financial Institutions to be the going rate of interest upon savings deposits in this state during the calendar year preceding the taxable year, such rate to be certified by the Commissioner of Financial Institutions to the Franchise Tax Board on or before the first day of March of each year.