Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.221
The term "actuarial method" as used in s. 422.201, Stats., shall mean the method by which that portion of each payment not applicable to an escrow account is applied first to any finance charge or permitted additional charge accrued from the time of any prior payment or from the time credit is extended and the remainder, if any, is applied to the unpaid amount financed. With the exception of the calculation of delinquency charges, amounts remitted may be applied to interest and charges and then to principal on the most delinquent installment due and then to interest and charges on the next installment proceeding to more current instalments until the amount remitted is exhausted. For purposes of computing the finance charge under s. 422.201(10m), Stats., a merchant may calculate the finance charge on an unpaid balance which includes unpaid finance charges.
Wis. Admin. Code Department of Financial Institutions DFI-WCA 1.221