The normal contribution rate shall be determined as the uniform and constant percentage of the annual salary of the average new permanent policeman entering the system which, if contributed on the basis of his salary throughout his entire period of active service would be sufficient, together with the assessment provided in N.H. Rev. Stat. § 103:7, to provide for the payment of any benefit payable on his account under this chapter. The accrued liability contribution shall be determined by the actuary as the amount necessary to liquidate the unfunded accrued liability as of June 30, 1968 over a period of 20 years from that date. The unfunded accrued liability is the amount of the total liabilities of the system which is not dischargeable by the funds in hand, the assessment and the normal contribution. After the determination of the unfunded accrued liability as of June 30, 1968, the normal contribution shall be determined after each actuarial valuation as the rate percent of the annual salary of each participating permanent policeman obtained by deducting from the total liabilities of the system the amount of the funds in hand, the present value of future assessments and the then current unfunded accrued liability and dividing the remainder by one percent of the present value of the future salaries of all permanent policemen who are then participating in the system.