The minimum amount of the refund credit on a retail installment contract to which Section 348.120 does not apply shall be computed in a manner proportionate to the method set out by that section for the type of motor vehicle being sold, having due regard for:
(1) the amount of each installment;
(2) the irregularity of the installment periods; and
(3) the requirements of Sections 348.104(f) and 348.106.