For purposes of this chapter, creditable compensation during the period of service in the uniformed services shall be computed as follows:
(a) The creditable compensation the member would have received for the eligible period of service had the member remained continuously employed.
(b) In the event the creditable compensation is not reasonably certain, creditable compensation shall be based on the member’s average creditable compensation 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.