(a) Any other law to the contrary notwithstanding, the board of trustees, for the purpose of administering the social security program, may, with due regard for the actuarial soundness of the retirement system and after an actuarial investigation into its benefit structure, adopt such tables, calculations, and compensation and deduction schedules as it shall deem desirable in connection with the proper operation of the retirement system. All tables, calculations, and compensation and deduction schedules adopted under this Code section shall be based on final monthly earnings, which shall mean monthly earnable compensation as reflected by monthly contributions during employment, except that no salary increase by adjustment in compensation in any manner during the last 12 months, which increase is in excess of 10 percent, shall be included. Such tables and calculations as may be adopted shall not diminish or impair the amount of any allowances or benefits which would have been payable to the member by use of tables or calculations that were in effect on January 1, 1959. The board of trustees may also, from time to time, establish rules and regulations for the administration of the funds created under this chapter and for the transaction of its business.

(b) 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.