The term maximum rate of interest as used in sections 4.1 and 4.5 of this Part, when applied to any loan or forbearance, secured primarily by an interest in real property, improved by a one- or two-family residence occupied by the owner, shall mean that annual percentage rate which will yield a sum equal to the amount of “interest” as defined in subdivision (a) of section 4.2 of this Part when such rate is applied to the unpaid balances of the amount financed, calculated according to the actuarial method (United States rule) by allocating payments between the amount financed and the amount of such “interest” so that a payment is applied first to such “interest” and the balance to the unpaid amount financed. The term amount financed as used in this section shall mean the sum of (a) the amount of the loan paid to, receivable by, or paid or payable for the account of the borrower, plus (b) such fees, charges or costs not included in “interest” as defined in subdivision (a) of section 4.2 of this Part but which at the time of any calculation of interest have been added to the loan or forbearance. Such term maximum rate of interest, when applied to any loan or forbearance not secured primarily by an interest in real property improved by a one- or two-family residence occupied by the owner, shall mean that annual percentage rate calculated according to any method authorized under New York law as it existed prior to the enactment of chapter 349 of the Laws of 1968.