(1" at breech, 3/4" at minimum and 13/16' at muzzle).
The top of your lock bolster and the top of your nosecap (if using a commercial one) will determine your stock line. It can be lowered slightly by placing your touch-hole lower.
If you make your own nosecap, pour one, or don't use one, then you can get the slimmest forestock by doing the following:
1. Lay a straightedge along the side flat of your barrel and place it so it is just above the bottom edge of the 3/4" section. That is your lowest possible point.
2. Draw a line along the straightedge (both sides of your barrel with a thinline marker.
3. Using that line at the breech, determine the lowest point near that line where you can position your touch-hole.
4. Inlet your lock. Then start removing wood until the line on your barrel shows. There may be a slight angle from your bolster to the middle of the barrel, but spread out over 24" it will be barely perceptable.
Obviously, this can't be done with a precarved, pre-inlet stock, as it all has to be layed out before starting the gun and everything placed and inlet in the proper position, and you have to make your own nosecap.