40 ILCS 5/12-124

"Fixation of annuity": As applied to a service annuity or prior service annuity or a surviving spouse's annuity, the final determination of the annuity at the date of retirement. A reversionary annuity calculated after January 1, 1990 may not be more than 75% of the service annuity granted to the employee annuitant on the date of retirement unless the minimum annuity to the surviving spouse payable under Section 12-135.1 exceeds the 75% maximum payable, in which case the minimum will be payable.