A. The annual disability benefit shall be equal to two percent of the member's average annual salary multiplied by the number of years of the member's total service-credit if the result is greater than one-third of the member's average annual salary. If the result of that formula is less than one-third of the member's average annual salary, the annual disability benefit shall be equal to the lesser of the following amounts:
(1) two percent of the member's average annual salary multiplied by the sum of the member's total service-credit plus the number of years, calculated to the nearest completed quarter, from the effective date of the member's disability to the member's sixtieth birthday; or
(2) one-third of the member's average annual salary.
B. A member's average annual salary for the purpose of computing disability benefits shall be the average salary for the last five years of employment or for any other consecutive five-year period for which contribution was made by the member, whichever is higher.
C. The annual disability benefit shall be paid in equal monthly installments.