You want to grind it when it's half frozen still anyway. Much easier to grind. Set it out to thaw. Rotate it by hand occasionally so that it thaws out more evenly, and then cut your chunks up and grind it.
You probably don't want to do a 50/50 mix. You probably want something more like 25lbs of venison and 8lbs of fat. .
An 80/20 or 70/30 is usually what I do for sausage.
-Jake