I have always went by just adding up tine length. IMO it takes 20 inches of tine length on a ten point to gross 120. On an eight point it should take around 25 inches. By this, i mean g1, g2, g3 for eight point....g1,g2,g3,g4 for ten point.

I'm sure there are better ways but this way seems to be pretty quick when it comes to a border line deer that I need to make up my mind. Usually takes about 10 seconds to add up the tines.