For purposes of computing service to award to members who die during qualified military service, service shall be computed as follows:
(a) The service credit the member would have received for the eligible period of service through the beginning of the period of qualified military leave through the date of death.
(b) In the event the service credit earnable is not reasonably certain, vesting credit shall be based on the member’s average service credit earned 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.