It's all a matter of taste. Dad never liked to mix any fat with venison to put in the freezer. Any fat would be added in the form of ground beef at the time of consumption. BOO! I was so dumb when I first made sausage I followed the same philosophy. I learned that in order to make sausage that was edible and not resembling sawdust, fat was required. I realize the OP is asking about burger and I will address that, but first sausage should be a ratio of at least 2:1 venison to pork butt which is 12% fat.
Now if you prefer your sausage to be a little more juicy the ratio can be adjusted to your taste.
As indicated earlier, in my family no fat was added to burger. The thought being that venison is the healthiest with no additives. And while true, I have no issues adding some fat, because overall, I am not consuming more fat in my diet to be concerned about whether or not it is high in cholesterol - the bad kind. If you are susceptible to or have high cholesterol, tailor your fat ratio to what is good for you. That said, let's talk about what is tasty!
I have found that 20% fat has more flavor and is more juicy. I think we tend to associate high fat content with cheaper less healthy BEEF HAMBURGER. I would like to make the case that if you add pork and beef fat from a known source (i.e good quality), that the end product will be better and possibly not as bad for you. I don't have any source to validate that, but if I use fat that I know where it came from, I feel better about it already. Fat is fat, yet "where did that fat come from" is always a question that bugs me if I buy a bag of beef tallow at the market or butcher.
So when I grind my burger, I use 2:1 venison to pork butt and then I add beef fat to make 15% or 20% fat content. It is a very small amount of beef fat, but it makes a world of difference! I save the trimmings from beef brisket for use in my burger. It is the best burger I have personally ground. We will have "burger steaks" frequently for dinner and believe you me, it is good eats!!
If anyone is interested, I have a burger "calculator" that will give you the proper weights of pork butt and beef fat based on the ratio of fat you want and the amount of venison you have (in pounds). IM me with your email address if anyone would like to have it.