NRI Flat Owner May Sell in Dec. 2023 which received in Feb. 2023 by Redeveloped Property

I am the original owner of a flat we purchased in .1973 Our building was redeveloped, and possession given in Feb. 2023. We may want to sell our flat now, but need to know if long-term or short-term capital gains would apply? We are not purchasing any other flat in India. We were told to wait 3 years by one CA. To avoid short-term CG, when can I sell? Also, what do I use as the cost-basis for determining capital gains: Original Purchase Price: Rs.27,500 (1973)---I do not have any idea what will be and how to get my cost as on 2021 (time of possession) valuation of property. Original Flat Square Feet: 481 Stamp Duty paid for of value (per redevelopment agreement): 24,26000 Net INCREASE in Flat Square Feet: 175 If I do have 3 year waiting period to avoid short-term capital gains, does this period start from the date of agreement with the builder (2021) or from the date of posession (Feb. 2023)? Would I have long-term capital gains on the original square footage and short-term on the % increase in square footage? For Example: If I sell for 1.2 Cr, what is my capital gains calculation?