As an alternative to the charges authorized by Section 18212 a licensee may contract for and receive charges at a rate not exceeding five-sixths of 1 percent per month plus a percentage per month equal to one-twelfth of the annual rate prevailing on the 25th day of the second month of the quarter preceding the quarter in which the loan is made as established by the Federal Reserve Bank of San Francisco on advances to member banks under Section 13 and 13a of the Federal Reserve Act as now in effect or hereafter from time to time amended, or if there is no

such single determinable rate for advances, the closest counterpart of such rate as shall be designated by the commissioner. Charges shall be calculated on the unpaid principal balance.