For purposes of calculating the employer’s liability under

Section or the required employee contributions under

Section 28001, the participant’s salary during that period shall be computed as follows: (

a) The salary the participant would have received with the employer for the eligible period of service in the uniformed services. (

b) In the event that salary is not reasonably certain, the contributions shall be based on the participant’s average salary with the employer during the 12-month period immediately preceding the eligible period of service in the uniformed services or, if shorter, the contributions shall be based on the participant’s average salary for the period of employment immediately preceding that period.