Start at 60 gr and work up 5 gr at a time. Accuracy will be different with each gun and dependent on other factors also like patch, lube ect.
Old School Rule-of-Thumb...., and since this is old tech, it's a pretty good starting point.
You start with grains of powder equal to your caliber rounded to the nearest five..., so that would be 58 grains round to the nearest five = 60 grains. So Mooman76 is right.
Then, I was taught for rifles, you test loads by ten (10) grain increments going upwards. You do this until a person standing 30 feet or so off to the side, can hear your bullet crack. A little less basic since powder isn't quite as precious as it was back in the day...,you test loads until you see the pattern open up, OR until you experience something disagreeable, such as too much recoil, or with a caplock if the hammer gets shot backwards to the half-cock position from back pressure through the nipple. Then to be precise..., you test five grains less and also five grains more.
Now in my state, the minimum deer load by the law was 60 grains of powder, regardless of the caliber, so when I started sighting in my .50 I started with 60 grains, instead of the 50 grains that the old rule specified. 70 grains worked great. When I upgraded to something larger in a .54....I simply tried the same powder charger for 70 grains, and that worked great for that ball too...., so I kept it.
Second Old School Rule of Thumb...., take your caliber, and multiply by 1.5. Round up to nearest five, and your pet load should be within +/- five grains of that. So 58 x 1.5 = 87...rounded = 90...so Your rifle should shoot well with a powder load of between 85-95 grains. That worked for me with the .50 which equaled 75 grains, and the 70 grain load was -5 under that. For the .54 though, multiplying by 1.5 = 81, so I should be shooting between 75-85 grains....oh well, they are "rules of thumb".
LD