A benefit for service retirement or reduced service retirement involving direct payments made by a vested inactive member shall be the option 1 benefit for service retirement, as calculated according to the formula set forth in section 24-51-603 or 24-51-605; except that the amount of the benefit shall then be multiplied by the ratio of service credit to the service credit required for eligibility as set forth in table B in section 24-51-602 or table C in section 24-51-604, whichever is applicable.