if you have a transmitter inside an open field, and you emit from 20m, and you take the same transmitter, same conditions, and put it inside a concrete building, and emit from 20 meters, through walls, you will get different dbm power values.
but you will get different power readings even in open field, for the same transmitter, at different times of day or while navigating around a 20m radius with the receiver.
ergo, the best way to determine these values are thorugh direct measurments, formulas can't apply here.