I'm assuming by FMV, you mean Fair Market Value? If so, keep in mind that FMV takes in consideration of any adjustments made to pricing (location, access, financing terms, zoning, etc.) , so you would first need to calculate the adjusted value of each lot.
Assuming you've already made the adjustments, and that all lots are of reasonable comparable size, then you could just divide the prices by square footage to come up with an average price per square foot and then apply that figure to the square footage of the lot for which you are attempting to determine FMV.
Oct 26, 2016