Fmv of inherited property on 01-04-1981

Respected sir, I had received a house in inheritance from my father in 1970 , now i sold it in Rs. 22 lakhs, In this case long term capital gain tax arise on me. In this matter i knows that actual cost of previous owner or FMV on 01-4-1981 should be taken as cost of acquisition and than get indexation benefit , but in this case house was also received in inheritance by my father their is no Previous owner cost , so only option to take FMV on 01-4-1981, but how to calculate the FMV of 01-4-1981 for the calculation of LTCG tax ? Please suggest .