Where the rate or amount of any item included in the total charge for credit or the amount of any repayment of credit under a transaction falls to be ascertained thereunder by reference to the level of any index or other factor in accordance with any formula specified therein, the rate or amount, as the case may be, shall be taken to be the rate or amount so ascertained, the formula being applied as if the level of such index or other factor subsisting at the date of the making of the agreement were that subsisting at the date by reference to which the formula is to be applied.