California Code, Education Code - EDC § 22853.5

(a) The compensation earnable that would have been applicable for the eligible period of service had the member remained continuously employed.

(b) In the event the compensation earnable that would have been applicable is not reasonably certain, compensation earnable shall be based on the member's compensation earnable 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 prorated over the period of service in the uniformed services.