The correct answer is 0!
I never have and never will believe in adding "pure fat" to venison. It's my guess that this idea was conceived by someone that was either to poor or to cheap to add quality ingredients to their venison.
If you've heard the cliche "garbage in, garbage out" - it holds true for making burger, venison or otherwise.
If you want a quality venison burger then and 25% of the venison weight in pork butt or pork picnic trim and 12.5% in ground beef chuck.
For 100# of venison that would equate to 25% of pork and 12.5# of venison.
I've used this formula and have received nothing but compliments for over 30 years.
Dave