Just use your cyl or imp cyl choke. You're on the right track to figure out ball diameter, but don't feel like you have to be too picky about the size. +/- .005" seem to work about the same for me. If the ball is a little oversize, it will just squeeze the plastic wad petals a bit. If it cuts the petals, the ball is too big, or too soft. Same applies to slugs.
I prefer WW, not pure; WW gives less (none) deformation in the forcing cone and better accuracy, although "better accuracy" is a very relative term in this case. You're basically shooting a smoothbore musket, with these round balls.
I honestly have not found better accuracy with normal rifled barrels, using round balls; the twist is much faster than a round ball wants.
