You can start at around 60 grains and work your way up in 5 grain increments to find out when you start to lose too much accuracy with patched round balls.
This willalso help youdetermine your best target load & hunting load with them.
If thatbarrel hasa 1 in 48 inch twist, you might want to consider trying different types ofprojectiles too.
Then you could work up an optimumload for each projectile at a common hunting distance for your area, say 50 or 75 yards, and then move it out to 100 yards to seeits actual long rangetrajectory.
You may want to consider trying outa sabot with a mediumlength bullet of about 240 - 250 grains. Sabots canproduce excellent 1st shot accuracy out of a cold, clean barrel. Then you'llprobably needto load more powder like cayugad mentioned, but you shouldswab the barrel between shots when testing for best accuracy and to simulatethe barrel's condition when hunting.
My son's 1 in 48 inchtwist, .50 caliber, 24 inch barrelshoots TC Break-O-Way sabots with a 240 grainplain lead hollowpointreally well loaded with 90 grainsof Pyrodex P.
Patched round balls should work well too, but you want to check for 1st shot consistency with a heavier powder charge out of the 24 inch barrel.