Where is her tag for. A few years back I drew a tag on the Island. After many scouting trips out there I found the best way for me to tell was from a side view. If the horn had a bit of a S shape to it the bull was closer to book size. I've taken a few people on Bison hunts since then and it seems to hold true. The Bison on the Island are a bit stunted from inbreeding so they will top around 1900lbs. The bulls around Yellowstone will go 2200lbs.
By the way, If you aim 6inch's behind the ear you won't ruin any meat, or have any tracking at all.
Regards Buff