(1) Class A: nine-tenths (⁄) of an allowance computed as a service retirement allowance on the basis of his average compensation and creditable service at the time of disability retirement, provided, however, that if his creditable service is less than twenty (20) years, a part or all of his additional years he would have had, had he continued in service to age sixty-five (65), may be added to his creditable service in order to determine his minimum retirement allowance, but in this event the total cannot exceed twenty (20) years, and for purposes of such determination, covered compensation at the time of disability retirement shall be deemed to have continued in effect during such additional years.
(2) Class B: nine-tenths (⁄) of an allowance computed as a service retirement allowance on the basis of his average compensation and average excess compensation and creditable service at the time of disability retirement, provided, however, that the disability allowance so computed shall not be less than twenty-five per cent (25%) of average compensation and average excess compensation.
(3) Class C: nine-tenths (⁄) of an allowance computed as a service retirement allowance on the basis of his average compensation and creditable service at the time of disability retirement but not less than twenty-five per cent (25%) of average compensation.