For summer sausage I do a 10 lb venison to 5 pounds pork.
For all other sausages I do a 6 lb venison to 4 pounds pork.
For bratwursts I do a 50/50 mix.
If a butcher ground your venison, there is a chance it is already cut with beef or pork fat, you may want to call him/her if you don't know.