For the purpose of computing benefits under this title, creditable service consists of: eligibility service as adjusted under subsection (b) of this section; and credit for unused sick leave as provided in § 20-206 of this article. This subsection does not apply to eligibility service that consists of: service credit transferred from another retirement or pension system; military service credit granted under Title 38 of this article; or service credit purchased under this subtitle. The Board of Trustees shall adjust a member’s eligibility service as provided in this subsection. The Board of Trustees shall adopt regulations to determine for any fiscal year the amount of creditable service that equals the eligibility service for a member who has completed less than the normal hours of employment for the member’s position. The normal hours of employment for a position equals the minimum number of hours to be completed by a full-time employee serving in the member’s position. The creditable service recognized for a fiscal year may not be less than the eligibility service for the member multiplied by a fraction, not to exceed one, that has: as its numerator, the number of hours of employment as a member that the member has completed during that year; and as its denominator, the normal number of hours for the member’s position.