California Code, Financial Code - FIN § 18636

(a) As an alternative to the provisions of Section 18635, if a loan is repayable in substantially equal and consecutive monthly installments of principal and charges combined, the first of which is due not less than 15 days nor more than one month and 15 days from the date the loan is made, a company may precompute charges and apply payments as provided in this article.

(b) The total charges which would be earned if the loan contract were repaid exactly according to its terms, at the monthly rate stated in the loan contract, may be precomputed when the loan is made and added to the principal of the loan. Every payment may be applied to the combined total of principal and precomputed charges until the loan contract is fully paid.

(c) The portion of the precomputed charge applicable to any particular monthly installment period shall bear the same ratio to the total precomputed charge, excluding any adjustment made for a first period of more or less than one month, as the balance scheduled to be outstanding during that monthly period bears to the sum of all monthly balances scheduled originally by the loan contract.