The refund credit on a contract to which Section 345.075 does not apply shall be computed in a manner proportionate to the method set out by that section, having due regard for:

1. (1) the amount of each installment; and
2. (2) the irregularity of the installment periods.

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