@Stone, its best to work up. At least it is for me. Do plan to take notes. Make only one change at a time.
I start by measuring the bore, both land to land and groove to groove. I then look at the number of grooves and how wide they are. For wide grooves and narrow lands, I use a ball that is 0.005" under the land to land measurement. If the lands and grooves are equal, I use a ball that is 0.010" under the land to land measurement. I use 100% cotton patches that I lubricate before loading. The patch thickness is the depth of the grooves plus 0.005" to try to give good grip to the ball and to fill the groove so gas cutting is minimized. Its better to go a few thousandths thicker but starting the ball and patch will become more difficult.
Prepare yourself to expect the firing. Shoot from a rest. Use targets that you can have a great sight picture so you can hold the rifle on target. Keep your sights shaded so light can't adversely affect the sight picture. Watch the wind.
The starting load will depend on the caliber. For 45 or larger, I start at the bore diameter and increase charges by 5 grains. For 40 caliber and smaller, I start with a charge that is half the bore diameter, 20 grains for the 40 and 15 for a 32 or 36. Shoot five shot groups. Record the results. Clean the barrel, chambered breech and touch hole. Increase the charge by 5 grains and shoot the next group. Do not exceed the maximum charge recommended for your rifle. Check your flint and if beginning to get dull, knap the edge to keep performance at its best. Adjust the flint for best striking position.
Velocity is not as important at this time as group size. Pressure can't be measured, so keep your loadings below maximum recommended loads.
As to the advantage of 3f versus 2f. One may have a smaller group at the sweet spot than the other. One may create less fouling. One may require less powder for best accuracy than the other. 3f will be a little faster when used as pan powder than 2f. 4F is faster in the pan than 3f or 2f but it is hard to actually difficult to feel the difference. I swear that I can sense a delay as I shoot my flint locks. When I watch the firing on video, the delay is not observable.