(a) The maximum finance charge included in a closed-end home improvement installment contract payable in substantially equal successive monthly installments beginning one month from the date the finance charge accrues, shall not exceed eight dollars ($8) per one hundred dollars per annum. Such finance charge shall be computed on the principal amount financed on the contract notwithstanding that the time balance is required to be paid in installments. The finance charge shall not accrue over a longer period than one which commences on the date of substantial completion of the contract and ends on the date when the final installment is payable. For a period less or greater than twelve months or for amounts less or greater than one hundred dollars ($100), the amount of the maximum finance charge shall be increased or decreased proportionately. A fractional monthly period of fifteen days or more may be considered a full month. If the finance charge computed as above provided is less than twelve dollars ($12), a minimum finance charge of twelve dollars ($12) may be made. With regard to open-end home improvement installment agreements, the maximum finance charge shall not exceed one and twenty-three one hundredths per cent on the outstanding balance from month to month.

(b) Subject to the limitations in subsection (c) of this section, when a contract is payable other than in substantially equal successive monthly installments, as where payable in irregular or unequal installments either in amount or periods thereof, or in regular installments followed by or interspersed with an irregular, unequal or larger installment or installments, or if the finance charge accrues from a date more than one month before the first installment is payable, the finance charge may not exceed an amount which, having due regard for the schedule of installment payments, will provide the same yield as if the contract were payable in accordance with the standard payment terms stated in subsection (a) hereof.

(c) If the amount of any installment is two times or more the amount of any other installment except the down payment, the amount of the finance charge in respect to the portion of the principal amount financed included in such larger installment shall not exceed the equivalent of six per cent per annum simple interest for the period from the date on which finance charge begins to accrue to the due date of such larger installment and such portion of the finance charge shall be payable in substantially equal periodic installments throughout such period.