I would say take off about 20-30 inches off of your estimates
No problems with your statement, but I would like to let you know that in all my field judging which resulted in an animal on the ground I am usually 5" short of the real score. Also I have been within 1" of every competition where you score the mount. I can see where you would say that, but my track record is much better than that. What units do you watch? I spend a huge amount of time on the manti-laSalles and the Anthro Range creek, way back in the boonies.