Our client recently sold mineral rights inherited in 1970s. We might have some information on its value through the estate tax valuation made originally at the time of inheritance, which is quite low.  I am reading there is something called a 36-48 month rule used in valuing mineral rights, based on the royalty income it has received (I have information on the historical incomes received). Is any one familiar with this rule? Trying to see if this method might give us a higher basis.