Let me see if I can help...I'm no expert, so someone may correst me, but i'll see if I can help you out.

First: There will always be 4 circumference measurements on an antler. One between the base and the G1, one between the G1 and the G2, another between the G2 and G3, and one between the G3 and G4. if the buck doesn't have a G4, you take it halfway between the end of the main beam and the G3. An extreme example would be on a spike. It still has 4 circumference measurements, all would be halfway between the base and the tip (they would all be the same because it doesn't have any tines).

And you are correct on the second part. You don't count the end of the main beam as a tine, because it is part of the main beam. An example would be on you basic 4x4, or 8 pt buck. It has 4 pts. per side. A browtine (G1), and two other tines (the G2 and G3). You count those lengths, and then the total length of the mainbeam, plus the other measurments. The end of the main beam counts as a point, bu not as a tine (ie. G4 in your case).

Hopefully, I helped explain this for you. If not let me know and hopefully we'll be able to figure it out.
Steven