Sec. 5.03. DETERMINATION OF AVERAGE MONTHLY SALARY. A member's average monthly salary is computed as the average of the member's compensation for the 36 months of highest compensation during the member's credited service. If a person has less than 36 months of credited service, the average monthly salary is computed, as if the member had been employed by the fire department for 36 months, by attributing to a period that is immediately before the member's employment and that is equal to the difference between the number of months the member has been employed by the fire department and 36 months of compensation the member would have received at the rank the member held when the person became a member.