Section 19. Notwithstanding the provisions of any premium finance agreement to the contrary, any insured may pay it in full at any time before maturity of the final installment of the balance thereof, and in so paying such balance shall receive a refund credit thereon for such anticipation. Such refund credit shall be computed on a method which is at least as favorable to the insured as the actuarial method, so-called. If the prepayment is made other than on an installment due date, it shall be deemed to have been made on the first installment due date if the payment is before that date, and in any other case it shall be deemed to have been made on the next preceding or next succeeding installment due date, whichever is nearer to the date of prepayment. Where the amount of the credit for anticipation of payment is less than one dollar, no refund need be made. The details of application and the rules for partial prepayments shall be subject to the regulations issued under the provisions of section one hundred and sixty-two B of chapter one hundred and seventy-five.