(1) The market value of approved class five pollution control equipment shall be determined by multiplying the depreciated value of the approved class five pollution control equipment in Montana by a market-to-book ratio.  The market-to-book ratio shall be determined by dividing the system or unit market value after deduction of the exempt intangible personal property by the system net book value after deduction of the exempt intangible personal property.  This value shall then be deducted from the Montana value and certified to the counties as class five property.

(2) This methodology shall be effective for all reporting years ending December 31, 1988 , and thereafter.