When I make deer-sausage, I use about 2lbs of venison, to 1lb of
el-cheapo fatty pork sausge(breakfast type). I usually by it and freeze it when its on sale. I then add in some additional breakfast sausage seasoning. I guess you could say it's 2/3 less fatty than pork sausage. We like it fine.
For just ground venison, I grind it and just leave it as is, and add trimings to when I cook it if I choose. I usually brown it in drippings or with a few strips of bacon.

I grind in 15% beef fat with my deer meat , works for me.

i don't see the purpose of mixing a 50/50 or even a 60/40 because if you like to eat deer meat then you like the taste of it-so why hide it so it tastes like pork or beef,so why do you bother to eat it. yes sometimes you'll get a deer thats alot gamier so i can see doing a 70/30 or even a 60/40 or for someone that has never tried it. i usually do a 80/20(plus getting as much deer fat out) at most-but you have to watch it closely when you cook it because it will cook and dry out faster.(i like med. rare to rare) but hey everyone will have their own opinion and taste. so i guess just experiment a little until you find what you like

try mixing in lipton dry onion soup mix,then grind and then when you thaw it out ,add 1 egg per pound to hold it together while frying.

I mix in 1lb.ground pork steak (often can find on sale for $0.99/pound) to 5 lbs venison (20% pork-to-venison ratio). I use about half that amount if making jerky because it drys out better and keeps the grease-factor down.

I'm with you on the 100% although I may not be a healthy eater. I just like the taste. For moisture, I mix chopped onion before grilling.