(a) A service retirement annuity is payable in monthly installments based on:
(1) the average monthly contribution during the member's term of qualified service with all participating departments under this subtitle, not including a contribution to reduce the unfunded actuarial accrued liability of the pension system; and
(2) a formula adopted by the state board by rule that allows the pension system to be maintained as actuarially sound.
(b) The state board by rule may provide, for each year of qualified service in excess of the period provided under Section 864.001 for full benefits, an additional amount that is a percentage of the person's monthly pension, compounded annually. A person may receive a proportional credit for months of qualified service that make up less than a year.