When grinding brisket for sausage or hamburger, I always cube it first, trimming and seperating the fat and lean. I find that most packer briskets are about 30-35% fat by weight.
I then weigh the fat and lean and mix the fat back in with the lean in whatever % I want. This time I wanted approx. 20-80, so, 1 lb fat to 4 lbs. lean.
Ready for the freezer.