California Code, Revenue and Taxation Code - RTC § 2132

(1) Determine the total amount of revenue required by the district which will be derived from an ad valorem tax or ad valorem assessment on property on the secured roll.

(2) Divide the amount determined pursuant to paragraph (1) among the counties in which the district is situated in the proportions that the estimated full values of taxable property on the secured roll within the district in the respective counties bear to the estimated total full value of such property within the district.