The minimum charge is always a variable, but in my book it's the one that gives you the best accuracy.
The idea is to experiment, each gun and barrel will have a bit of a different charge for accuracy because each individual shooter will have his/her own style or technique for loading things as small as tapping the powder measure and how hard you push the final time can change things, don't worry too much what you do will be good for you.
Start at about 50-55grns, shoot 5 rounds at a target aiming at the same point, then increase the powder charge 5 grns and shoot 5 more times at a clean target. Continue the 5grn increases up too around 80-90grns.(Max is seldom accurate)
It helps to clean the barrel between these charge changes, kinda give each charge a fair starting point.
Somewhere in there you'll find one target that has a better group than the others.
It'll take some shooting to find it, but a "hunting load" is one that will place the ball in the kill zone accurately and repeatably.
A 50cal rifle with a round ball is plenty for Deer and Blackbear size game, Many states require a .40 cal or more for Deer, the 50 is plenty