Any person participating in a retirement plan authorized under this subchapter on or after July 1, 1999, shall be entitled to a retirement benefit if that person has ten (10) or more years of actual service in a plan covered by this subchapter and is at least sixty (60) years of age. If the person does not have sufficient service to draw the full benefit provided, then a partial benefit not to exceed the full benefit shall be payable when the person has reached the age requirement specified in this subchapter. The partial benefit shall be determined by multiplying the full benefit by the following fraction:
(1) The numerator shall be the number of months of actual service under the provisions of this subchapter; and
(2) The denominator shall be two hundred forty (240) months.