Now the reverse-engineering. If your HRA is say 24% of Basic salary. Then add 10% of basic on it. So a Basic salary of 90k can have a rent p.m. of about 30k per month and thus the HRA of 21k per month will whole come out to be exempted. If you live in Delhi, Mumbai Kolkata or Chennai then do not exceed you HRA above 50% of Basic. If you live anywhere else, then limit it at 40% of Basic Salary. The (base)rent for the above can be calculated at 60% of Basic and 50% of Basic respectively. Remember that amount should also fulfill above condition