70/30 or 75/25 or even 80/20 will work. Since venison is like uber lean to start with, you can vary the fat of the pork by how you trim up the butts. I've not used the
Excalibur mix so I can't comment on that. If you have Amesphos or Butcher & Packer "special binder", I would add that as well. I've used it my last few batches and the texture was very nice and moist and it leaves less to chance depending on your meat/fat ratio to start with. It does not affect the flavor just the texture and mouth feel (in a good way IMO). Go by the ratio recommended for what you have.
I usually use the B&P #414 special meat binder. It's 2 ounces to 25 pounds of meat, which works out to 0.08 ounces per pound or 0.8 ounces for a 10 pound batch (assuming your 7 pounds of venison and 3 pounds of pork). If you have Amesphos, it is 1/4 to 1/2 teaspoon per pound of meat for sausage making. Note: if the
Excalibur mix has any soy or milk proteins in it already, I would cut back on the B&P or Amesphos. Might even cut it in half.
I like to buy several extra pork butts when they are on sale, and cut them into 1"x1" by however long strips. Then I vacuum pack and freeze the strips laying flat for grinding later. The 1" wide strips just flow into my grinder and I can make pretty much anything from pork butt. At $0.75 a pound I would have filled the freezer!!!! (and left a few whole for pulled pork also).