A trespasser is liable to account for the profits which he makes and which he would have made with ordinary diligence but not for profits derived by him due to improvement effected by him e.g., digging a well.
The test in such cases is not what the plaintiff has lost by being out of possession but what the defendant has made or might reasonably or with ordinary prudence have made by his wrongful possession.
As stated above, a person in wrongful possession is not liable for failure to realize the highest possible rates of rent but only a fair rent. So when the person in wrongful possession planted indigo for use in his adjacent factory and it was proved that an ordinary farmer would have grown sugarcane, wheat or tobacco, their Lordships of Privy Council in Gray v. Bhagumian, [(1939) 57 I.A. 105] assessed mesne profits on the profits of cultivation of those more profitable crops.
The determination of the amount rests entirely at the discretion of the court and it is difficult to lay down fixed principles which can be applied in every case.