The CTC is based on basic etc and not the other way around.
Yet if your basic pay is Rs X (this is decided by your employer), your HRA will be 40% of this basic (if you reside in a non-metro city) or 50% (if you reside in any of the 4 metros). Medical, as already pointed out is fixed at 15000 per year.
This will not be precise, but in case you reside in a non-metro and with 12% as PF contribution of employee as well as employer, your basic should be 13567 a month for a gross of 23500; HRA - 5426 and medical - 1250,PF - 164 of employee and employer.


HRA exemption is depends on Basic Salary so you can fix your basic salary based on House Rent you are paying, medical is fixed Rs.15000/-