The actuary shall investigate the plan's claim experience, as provided by sections 508 and 517 of the Education Law. On the basis of such investigations and upon the recommendation of the actuary, the retirement board shall certify the premium rates computed to be necessary to fund the group term life insurance authorized to be paid by the plan. As soon as practicable after the close of each fiscal year, the retirement board shall determine the premium which the employers participating in the system are required to pay into the reserve fund to discharge the obligations of the plan of the past fiscal year.