In this section, “employees’ systems” means the Employees’ Pension System and the Employees’ Retirement System. As part of each actuarial valuation, the actuary shall determine the normal contributions, net of member contributions, on account of members of the employees’ systems who are employees of participating governmental units. Except as provided in subsection (d) of this section, the normal contribution rate for participating governmental units equals the fraction that has: as its numerator, the sum of the normal contributions determined under this section; and as its denominator, the aggregate annual earnable compensation of the members of the employees’ systems who are employees of participating governmental units. On the recommendation of the actuary, the Board of Trustees may adjust the normal contribution rate to reflect: experience gains and losses; the effect of changes in actuarial assumptions; and the effect of legislation enacted after July 1, 1980.