Good question.
I just buy trimmed pork back fat and go 80% venison meat 20% pork fat. I make amazing sausage with it all the time. Should work for sticks.
I take the approach that I want as much venison in my final product as possible and only add the minimal amount of other stuff which in this case would be fat.
As for pork butts, they are already about 80% meat and 20% fat so when you add venison you basically skew the meat to fat ratio since venison has no real fat plus you wont like the flavor of venison fat so just go with the meat.
I read of guys mixing like 50/50 venison meat with pork butt, which always makes me really wonder how well their stuff comes out. It could come out just fine and only doing it would tell but conventional sausage meat to fat ration are from 70/30 to 85/15 so I'm a little skeptical on 50/50 porkbutt to venison combos.
Someone who does pork butt + venison will chime in for sure but this is some food for thought on how you may want to approach it :)