According to the Hodgdon reloading manual, start a 50 grains, and go up to 70 grains of FFg in that .58. Remember that you are shooting a heavy ball, and its going to keep on going once you get it started. It may start out slow, but it will lose less velocity than a smaller ball going faster. Figure out a good paper load, for 25 and 50 yard shooting, and then work up a HELLO load for hunting and to shoot at 100 yards. The ball will kill at many times that range, but the trajectory of a round ball is such that it begins to drop off very fast not much beyond that 100 yard mark.
You want a hunting load that has a point blank trajectory of no more than 3 inches above the line of sight, and no lower than 3 inches below the line of sight( Point of aim) either between the muzzle and the zero range distance, and a calculated distance beyond your zero distance.
For instance, I zero my .50 caliber rifle to put the ball 2 inches high at 50 yards, and 1 inch low at 100 yards. At 125 yards, it drops that next inch, so my point blank range for that gun and load is plus or minus 2 inches to 125 yards. If I were to zero the gun at 100 yards. that would put the ball 3 inches high at 50 yards, and 3 inches low at about 140 yards. That is a very long shot for me using open sights. I want to be closer, and I usually can get closer, if I see the deer in time to move.