(2) No licensee may make a loan using the add-on method to calculate interest that does not provide for a refund to the borrower or a credit to the borrower's account of any unearned interest when the loan is repaid before the original maturity date in full by cash, by a new loan, by refinancing, or otherwise before the final due date. The refund must be calculated using the actuarial method, unless a sum equal to two or more installments has been prepaid and the account is not in arrears and continues to be paid ahead, in which case the interest on the account must be recalculated by the simple interest method with the refund of unearned interest made as if the loan had been made using the simple interest method. When computing an actuarial refund, the lender may round the annual rate used to the nearest quarter of one percent. In computing a required refund of unearned interest, a prepayment made on or before the fifteenth day after the scheduled payment date is deemed to have been made on the payment date preceding the prepayment. In the case of prepayment before the first installment due date, the company may retain an amount not to exceed one-thirtieth of the first month's interest charge for each day between the origination date of the loan and the actual date of prepayment.