How to calculate capital gains on selling a self built building

I am planning to sell a flat in the next month for 40 lakhs. This house having 3 floors was built in year 2002 on a plot purchased at 2 lakhs. The approximate cost to build a house was about 10 Lakhs, however I don't have any receipts for the same. But 1 floor was sold in 2004 for 4 lakhs for which i have the documents available.

I would appreciate your help on clearing following points How do I calculate the indexed value of this house? Can i use the 4 lakhs of 2004 as the basis for indexing? Or While mentioning the acquisition cost on IT form, do I have to provide the receipts to justify the cost of building the house? How do I show my capital gain in IT return? I am planning to buy NHAI bonds worth [selling price (40 L) - indexed cost (12L)] to save capital gains tax, would that be a suitable way to proceed?

Thanks

-Chinmaya