“Actuarial tables”: Such tabular listings of assumed rates of death, disability, retirement and withdrawal from service and mathematical functions derived from such rates combined with an assumed rate of interest, based upon the experience of the system, as adopted by the board upon recommendation by the actuary.
     The adopted actuarial tables shall be used to determine the amount of all benefits under this Article, including any optional forms of benefits. Optional forms of benefits must be the actuarial equivalent of the normal benefit payable under this Article.