Hi RK,
Don't change SAP standard functionality for HRA tax exemption calculation as per govt rule.
As per communication had with other fellows i will suggest you tell to your client to maintain metro citiies instead of non metro city in IT581 and do small enhancement in the IT581 that create one extra field where you will maiantine exact city type as per your reguirement.
It's good practise.
Regards,
Sankarsan