(a) A supplemental disability retirement annuity payable under this subtitle is an amount that, when added to a member's standard disability retirement annuity or, if the member is eligible for service retirement, to the member's standard service retirement annuity, equals one-half of the member's average monthly compensation for service as an employee of a participating department of a municipality:
(1) for the 60 months immediately preceding the month in which the injury occurred; or
(2) if the member did not perform service in each of the 60 months immediately preceding the month in which the injury occurred, for the number of months of service within the 60-month period.
(b) In a computation of average monthly compensation under this section, compensation is excluded that exceeds the maximum amount on which the member was required to make contributions to the retirement system.