The program payment shall be calculated by the system as an aggregate amount equal to a percentage of the judge’s monthly salary for each month of the extended service period, taking into account any salary increases occurring during the period, plus monthly interest thereon at a rate indexed to 30 year United States Treasury Bonds. For the first to the 60th month, inclusive, of the extended service period, the calculation amount shall be 20 percent of the judge’s monthly salary. For the 61st to the 120th month, inclusive, of the extended service period, the calculation amount shall be 8 percent of the judge’s monthly salary.