When you have production guns or barrels from known manufacturers, going 0.010" or 0.005" under the published spec is a reasonable approach to selecting ball size. However I have bought several guns that don't have marked caliber on the barrel. My T. Strubel flintlock pistol being one and other measures have to be adopted. What will happen to all those T/C barrels sent off to Bobby Hoyt and now they are some other caliber than the one stamped on the barrel. Then there are those coned barrels that will be much larger at the muzzle than further down the barrel.
I inherited my tool maker father's tools, I have all the calipers, micrometers, telescoping gauges and ball gauges, so measuring bores was not much of a problem. I do understand that such measurements can be a problem for some. Well, that's why we are on these boards to try to help as best we can.
A trial and error approach can work with an assortment of balls of different diameters. Drill a hole through the ball and insert a bolt and a nut to hold the ball on the bolt. the long threaded end can be pulled up to remove the ball. When you have a good slip fit and little side play you can try loading the bolt and ball with a patch to see how that feels. Now you have a starting point for load development. Pin gauges are another choice, but can be expensive. A set of drill bits can be used for an approximate size, but they are not exact enough to determine if you need a ball 0.005" different. And if the bore is larger than 0.500" drill bits aren't the best option. There are those tapered bore gauges that can help but they are not exact enough either.