Testing (in air) from a small gold coin through to a one oz. one I get readings of 62, 68, 72 and 78. Angle of coin changes the reading and the ground conditions. The more mineralised, the lower the number is.
Put say 25 one oz. coins in a stack and sweep across the pile and the number can be as low as 11 ! Tape stack together and lay on its side and one direction of sweep indicates 28, sweeping the other direction 49.
For gold you have to bite the bullet and dig everything above small iron.