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. (1) the amount of each installment;
2. (2) the irregularity of the installment periods; and
3. (3) the requirements of Sections 348.104(f) and 348.106.

Acts 1997, 75th Leg., ch. 1008, Sec. 1, eff. Sept. 1, 1997.