Newbie question.
Is there a rule of thumb for what size ball to use in any given calibre.
Yes I would say there is a rule of thumb for determining the ball size required by the firearm.
The rule would be to use a ball of sufficient diameter so that when you slug the bore with a patched ball then remove that ball to examine it. Look for the patch cloth weave marks deeply embossed into the ball surface where the patch was squeezed hard between the rifling lands and the ball surface.
Also you should see weave marks lightly embossed on the ball in the groove areas.
On my Lyman GPR 54 I tested a .530" ball with my chosen patch material. That ball size did not show cloth marks embossed all the way around the circumference of the ball. I even use a magnifying glass to examine the ball, there was no trace of the patch weave in the groove areas of the rifling.
Slugging the barrel with the .535" ball showed the weave marks all the way around the balls circumference.
I choose to use the larger .535" ball and test firing proved it to be the more accurate of the two sizes.
I can load the smaller ball much easier but I prefer the tighter shot grouping I get with the larger ball for fine target work and that is what the rifle gets used for 98% of the time.
I could have chosen to fiddle with patch thickness to make either ball work, but I choose to start with the patch material first. The 100% cotton cloth I choose was base on being not to thick and not to thin and had a very tight weave and was commonly stocked at the fabric store.
The patch cloth was double washed but not heat dried which would cause shrinkage (I could always go back and shrink in the dryer if the need arose) then ironed flat.
Even though I choose to use the larger ball I have happily noticed that when I need to unload the rifle without firing, the larger ball slides up the bore very easily. Much easier than when it when down the bore (clean). I suspect that the soft lead ball continues to distort while under pressure of the patch and rifling till a sort of equilibrium it reach.