Bobvilla it is an obsession, ain't it?
Pork butt is around 70/30 on average. That alone makes a good sausage. But if you mix in venison, here's 2 schools of thought.
1. Go with what you have been using. You say it has been working out quite well. Consider sticking with what works. It's your family that likes it, right? Kinds like ribs, competition says it should have a gentle tug off the bone. My family says fall off the bone. I give 'em what they ask for. Besides, I like it too.
2. My recipe for venison sausage is 50/50/30. That means equal weights of pork and venison. Weigh that, and add in 30% more pork back fat.
So, for 10 lbs meat, you would use 5 lbs port butt, 5 lbs venison, and 3 lbs back fat. HEB can order it, Asian markets usually carry it. Figure your cure (if you use it) for 13 lbs, or 5.91 KG of product. This will give you a great sausage medium that isnt too fatty but has enough in it to gel up well.
I think skinnerback would like it