Here is a quick slide show on a packer I did while back. trimmed almost all the exterior and big chuncks of fat off. still had plenty of fat for great burgers. it's real easy to do if you have the kitchenaid grinder attachement. Somewhere I have a write up with yields and such, did two packers.
One interesting thing about hamburger, I've asked several butchers how they get to the 80/20, 85/15 or 90/10 ratios. I'm thinking a scientific fat measuring device or by using select meat cuts or at least some big secret technique. All said, they guess. t