Mich. Comp. Laws § 460.113

Sec. 3. For the fiscal year commencing July 1, 1973 and annually thereafter, there shall be deducted from any amount to be assessed under section 2 an amount equal to the difference by which the actual expenditures for the previous fiscal year attributable to the regulation of public utilities are less than the amounts appropriated for those purposes. Such deductions shall be made in the same proportion as the original assessments in section 2 of the act.