The retirement benefit per annum to any person retiring under the provisions of this chapter shall be two and one-half per cent (2 ½%) of the average compensation for each year of service (but in computing, not to exceed twenty-four (24) years service) provided said benefit shall not exceed sixty per cent (60%) of said average compensation. In computing length of time of service, fractions of a year equaling six (6) months or more shall be treated as a full year of service.
T.C.A. § 8-4016