(a) For purposes of transportation aid payable pursuant to Education Law, section 3602(7)(a), approved retirement expenses shall be determined by multiplying by the ratio of other aidable transportation expenses to total transportation expenses, exclusive of retirement expenses:

1. (1) the district share of retirement expense payable to the New York State Teachers' Retirement System in the base year for transportation personnel;
2. (2) the district share of retirement expense payable to the New York State Employees' Retirement System in the base year for transportation personnel;
3. (3) the district share of retirement expense payable to the New York City Teachers' Retirement System in the base year for transportation personnel;
4. (4) the district share of retirement expense payable to the New York City Employees' Retirement System in the base year for transportation personnel; and
5. (5) the district share of retirement expense payable to the New York City Board of Education Retirement System in the base year for transportation personnel.