For purposes of computing an employer’s contributions for the eligible period of service or the member’s contributions under this part, the employee’s compensation earnable during the period shall be computed as follows:
(a) The compensation earnable the member would have received for the eligible period of service.
(b) In the event the compensation earnable is not reasonably certain, the employer’s contributions and member’s contributions shall be based on the member’s average compensation earnable during the 12-month period immediately preceding the eligible period of service in the uniformed services or, if shorter, the period of employment immediately preceding that period of service.