(a) On and after April 1, 1964, the board of trustees is authorized to adopt simplified benefit tables which will enable a member to estimate his or her retirement allowances. Such tables shall (1) be based on an actuarial study, (2) maintain the actuarial soundness of the retirement system, (3) for those members retiring on and after April 1, 1968, be applied to the member's highest average monthly earnable compensation during a period of 24 consecutive calendar months while a member of the retirement system, and (4) be applicable to all members, provided that the application of such tables shall not reduce or impair the amount of any allowances or benefits to which any person who was a member on April 1, 1964, would have been entitled at that time or would be entitled at any time thereafter under tables or calculations which were in effect at that time or at any time prior thereto or at any time prior to the adoption of such simplified benefit tables.

(b) The regular service retirement allowance payable to a member pursuant to the provisions of this chapter, prior to the application of any minimum benefit formula otherwise provided under this chapter, shall be determined pursuant to the formula adopted from time to time by the board of trustees for such purpose. Such formula shall be uniformly applicable to all members similarly situated. The board of trustees may establish rules and administrative procedures uniformly applicable to all members similarly situated relating to the calculation of such service retirement allowance.