The two scales agree for D = 6 inches.  If you have large logs whose diameters are bigger than 6 inches, then you would be looking for the largest board volume.  For example, if the diameter was 12 inches, then The Doyle Scale would give you  (12-4) squared or 64 board feet.  On the other hand, The Scribner Rule would give you 144 - 24 - 20 or 100 board feet.        Return