What do you mean by reverse indexation method? it's always a forward calculation. Nevertheless, for academic purposes, take FMV as of today, multiple by 100 and divide by 280 (2018-19 CII). That will take you to the FMV in 2001. If your question is simply how to know the FMV of a property in 2001, you can check with your registrar of properties of your location to know the circle rates that were prevailing then. You can also check with a professional property valuer on the rates of 2001 of similar property.