Any member who becomes incapacitated for performance of duty because of disability determined by the retirement board to be of extended and uncertain duration, and who shall have completed at least 10 years of service credited in the aggregate, computed as provided in Section 8.584-7, shall be retired upon an allowance of 1½ percent of the average final compensation of said member, as defined in Section 8.584-1 for each year of credited service, if such retirement allowance exceeds one-third of his average final compensation; otherwise 1½ percent of his average final compensation multiplied by the number of years of City service which would be credited to him were such City service to continue until attainment by him of age 60, but such retirement allowance shall not exceed one-third of such average final compensation. In the calculation under this section of the retirement allowance of a member having credit for service in a position in the evening schools and service in any other position, separate retirement allowances shall be calculated, in the manner prescribed, for each class of service, the average final compensation in each case being that for the respective class of service; provided that the average final compensation upon which the minimum total retirement allowance is calculated in such case shall be based on the compensation earnable by the member in the classes of service rendered by him during the three years immediately preceding his retirement. Part-time service and compensation shall be reduced to full-time service and compensation in the manner prescribed by the Board of Supervisors, and when so reduced shall be applied as full-time service and compensation in the calculation of retirement allowances. The question of retiring a member under this subsection may be brought before the retirement board on said board’s own motion, by recommendation of any commission or board, or by said member or his guardian. If his disability shall cease, his retirement allowance shall cease, and he shall be restored to service in the position or classification he occupied at the time of his retirement.