The composite interest rate for the new mortgage debt shall be a combination of the original mortgage interest rate and the prevailing interest rate and shall be calculated as follows:

1. (1) The original mortgage interest rate shall be multiplied by a fraction, the numerator of which shall be the existing principal balance and the denominator of which shall be the existing principal balance plus the additional principal.
2. (2) The prevailing interest rate shall be multiplied by a fraction, the numerator of which shall be the additional principal and the denominator of which shall be the existing principal balance plus the additional principal.
3. (3) The sum of subdivisions (1) and (2) of this section, rounded to the nearest one-eighth of one per cent, shall be the composite interest rate.

(Effective August 27, 1984; Amended February 9, 2009)