Greenhill Formula
Where:
D = bullet diameter in inches
L= bullet length in inches
150 = a constant
This is set up for muzzle loaders for cartridge guns that shoot 2800 FPS or more use 180 as the constant.
The barrel length has to do with tradition and the barrel length that real black powder is most efficient at with a round ball.
To kind of give you an idea for muzzle-loaders
with a bullet 1 caliber in length a twist of 1in 48 to 1 in 80 is recommendable
with a bullet 1.5 calibers in length 1 in 48 to 1 in 38
with a bullet 1.7 to 1.9 calibers in length 1in 32 to1 in 28
This is just to try and give you a rough idea as there are many exceptions as barrel whip can enter in to it and even a smooth bore will sometimes shoot quite well to 75 or so yds.
Hope this helps you understand. Lee