.562 sounds about right
I don't have one, but it depends on how tight or loose you want the load to be
and your exact bore size.
Generally, it's said that you use a .02 smaller ball for a smoothbore (unlike a rifle) and use a thicker patch. Unlike using tighter fitting balls in a rifle, this allows for loading & shooting more balls without needing to swab as often because there are no rifling grooves to hold the powder fouling in a smoothbore.
And by using the tighter ball and patch combination, you may be able to get better accuracy, but
will have to swab more often.
The .570 ball may be too tight with a .018 pillow ticking patch if you have a true .58 bore, so you would have to go to a thinner patch like a .015 or less. And if you go too thin, you're going to get burn through (patch failure) or you're going to need an overpowder wad of some kind. Or you might have to hammer it in.
Another option would be to use a .550 ball and the .018 pillow ticking. It may or may not be snug enough, but you can always increase the patch thickness to .020 or more if needed. Some folks use denim patching with large caliber smoothbores too, depending on the tightness desired.
The math would work out this way:
You would double (2X) the thickness of the .018 patch and add that to the .550 ball diameter and it would equal (.550 + .036) .586.
Track of the Wolf sells .562 balls, and those would seem to fit the original premise of using balls .020 under bore size in a smoothbore. Would you be willing to use a .015 patch if necessary?
But everything depends on the exact bore diameter of your barrel. With smoothbores, there aren't any rifling grooves to take up the extra patching thickness either, so it's all about how tight that you want the loading and ramming to be, the amount of swabbing per shots taken and the degree of accuracy desired.
http://www.trackofthewolf.com/(S(cn...ageSize=25&Page=3&Search=lead balls&Type=Desc