The total amount payable in each year to the accumulation account shall be not less than the sum of the rate per cent known as the normal contribution rate and the accrued liability contribution rate of the total compensation earnable by all members during the preceding year.

The accrued liability contribution shall be discontinued as soon as the accumulated reserve in the accumulation account shall equal to the present value, as actuarily computed and approved by the board of trustees, of the total liability of such fund less the present value, computed on the basis of the normal contribution rate then in force, of the prospective normal contributions to be received on account of all persons who are at the time members.