This section applies only to members of: the Correctional Officers’ Retirement System; the Employees’ Retirement System; and the Teachers’ Retirement System. An ordinary disability retirement allowance equals the greater of: a normal service retirement allowance; or the amount computed under subsection (c) or (d) of this section. If a member is at least normal retirement age, the amount to be used under subsection (b)(2) of this section is 25% of the member’s average final compensation. If a member is under normal retirement age, the amount to be used under subsection (b)(2) of this section is the lesser of: 25% of the member’s average final compensation; or the normal service retirement allowance that is computed by using: the number of years of creditable service that the member would have received if the member continued employment until normal retirement age; and an average final compensation that the member would have received if the member continued employment without a change in earnable compensation.