Sec. 17. Beginning with the third year of the trust, and each year after that year, the unitrust amount for a current valuation year of the trust is the product of the unitrust rate multiplied by the average of the net fair market values of the assets held in the trust on the first business day of:
(1) the current valuation year; and
(2) each of the two (2) years of the trust immediately preceding the current valuation year.