Mich. Comp. Laws § 38.2215

Sec. 215. The retirement board and the department shall establish the method of determining the actuarial cost. The actuary shall calculate the actuarial cost based upon the product of the following elements: (a) A percentage that when multiplied by a member's compensation 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 the member's age, credited service, or plan member classification or the retirement allowance beneficiary's age. (b) A member's compensation. (c) The number of years and fraction of a year of service a member elects to purchase up to the maximum, if any.