87.105. During a senior administrator’s progressive pre-retirement, the pensionable salary for the years or parts thereof covered by the agreement for the purposes of the retirement plan is the salary that the senior administrator would have received had he not availed himself of progressive pre-retirement or would have been entitled to receive for a period in which he receives salary insurance benefits. The service credited is the service that would have been credited to him had he not availed himself of progressive pre-retirement.