Suppose CII for 2018-19 would be 286
Full value of consideration Rs. 25,00,000
Indexed COA Rs. 10,03,860
Capital Gain would be Rs.14,96,140/- and tax would be Rs. 3,11,200
Note: Considering COA as Rs.3,51,000. You should obtain FMV as on 01.04.2001 which will substantially reduce the capital gain amount.
As you sold flat in the F.Y. 2018-19 (i.e. A.Y. 2019-20) and in the starting month i.e. April 2018, therefore you have ample time to deposit in capital gain account scheme. You can deposit the amount before 31.07.2019 subject to your reinvestment plan.
If you want to reinvest in another house property you should purchase another property within 1 year from the date of sale or construct within 3 years from the date of sale.
It is better to deposit in the CGAS otherwise you need to pay Advance tax in three installments.