In all other consumer credit transactions according to the rule
of 78ths or sum
of the digits method, meaning the amount
of the refund or credit shall be as great a
proportion of the
finance charge originally contracted for as the sum
of the periodic time balances
of the
debt scheduled to follow the date
of prepayment bears to the sum
of all the periodic time balances
of the
debt, both sums to be determined according to the scheduled payments originally contracted for.