Mich. Comp. Laws § 38.1379

Sec. 79. (1) Actuarial cost shall be equal to the product of subdivisions (a), (b), and (c): (a) A percentage, determined by the retirement board and the department, which when multiplied by a member's compensation, as determined under subdivision (b), results in the average actuarial present value of the additional benefits resulting from the crediting of 1 additional year of service. The percentage may vary because of age, credited service, or benefit coverage. An increase or decrease in the percentage under this subdivision shall not become effective before the expiration of 6 months or more after the retirement board notifies the reporting units of the increase or decrease. (b) A member's compensation. The member's compensation shall be the member's full-time or equated full-time compensation earned in the school fiscal year immediately before the school fiscal year in which the application to purchase and payment for the service are made. The compensation amount used shall not be less than the highest compensation previously earned by the member. (c) The number of years, including any fraction of a year, of credited service a member elects to purchase up to the maximum allowed. (2) If the compensation amount used for computing payment under this section exceeds the member's final average compensation determined at the time of retirement, the payment required under this section shall be recomputed using the member's final average compensation and a refund shall be made based upon the recomputation.