Sec. 13. (a) If only a part of:
(1) a tract or lot of land; or
(2) a building;
connected to the sewerage system is exempt from general taxation, the rates and charges established under this chapter shall be reduced by a fraction.
(b) The fraction to be used under subsection (a) is established by using:
(1) the assessed valuation of the part subject to tax as the numerator; and
(2) the total assessed value as the denominator.