40 ILCS 5/15-154.1
The combined retirement annuity, survivors insurance, death benefits, or disability benefits under this system and any other system which is, has been, or may be financed fully or in part by contributions from any alumni association, foundation or athletic association affiliated with the universities included as employers, shall not exceed the largest benefit which could be payable by this system or such other system based upon the participant's combined service and earnings credits.
If the combined benefits exceed the largest benefit as determined in accordance with this Section, the benefits payable by this system shall be reduced to that amount which, when added to the benefits payable under such other system, equals this largest benefit.
If benefits are reduced, this system shall pay to the alumni association, foundation or athletic association which financed such other system, that fraction of the reduction, the numerator of which is the amount of the benefits payable by the other system prior to the adjustment and the denominator of which is the amount of the combined benefits which would have been payable prior to the adjustment. In determining the adjustment of the survivors insurance or death benefit made under this Section, the benefits payable under this system and the other systems shall be reduced to common actuarial equivalents in accordance with the mortality, interest, and annuity tables adopted by the board and the method of payment required by this Article.
40 ILCS 5/15-154.1