Why HRA trips people up
House Rent Allowance exemption is not a percentage of anything. For FY 2025-26 it sits in section 10(13A) of the 1961 Act; from FY 2026-27 in Schedule III of the Income-tax Act, 2025, read with Rule 279 of the Income-tax Rules, 2026. The formula is the same under both. It is the least of three separate amounts: the HRA your employer actually pays you, half your basic salary if you live in a metro or 40% if you do not, and the rent you paid less 10% of your basic salary. Whichever of those three is smallest is your exemption, and the other two are irrelevant.
That structure is why two people with identical salaries can have wildly different exemptions, and why raising the HRA component of your salary sometimes changes nothing at all. If the third amount is the smallest, the only thing that moves your exemption is the rent you actually pay.