@PropanAbuser, since you are measuring metal, putting excess pressure on your calipers may add a few thousandths to your reading. On metal I like a slip fit with the calipers.
Dutch Schoultz (the author of "Black Powder Accuracy") and I have had a difference of opinion on the pressure one applies with the measuring device on patching material. He liked to get the compressed measure and he applied quite a bit of pressure. In my opinion, I wanted a slight amount of compression to see if the material would compress, but not so much as to abuse a precision measuring tool. There is no way that I would apply enough pressure to compress fabric to the 0.005" or 0.0025" between the lands and the ball. Besides if one uses a pure lead ball, the patch will engrave the ball between the lands and the ball. I do recommend that new shooters and owners of their first muzzleloading firearm get Dutch's System. Even if I may have a difference of opinion on some topics and he focuses on percussion firearms, the information with respect to load development and care of a muzzleloading firearm is worth the cost of the 94 pages of more information than I would want to try to place on the Forum.
http://blackpowderrifleaccuracy.com/
Now for my thoughts on the ball size for your pistol. Verify that you have a smoothbore. Let's see the muzzle. In any event, you should start out with an easy to load patch and ball. I would get 0.440" balls. I would also get unlubricated pre-cut patches that are 0.015" thick to be lubricated right before loading. These patches will compress slightly while minimizing gas cutting. Lubricate the patch with any of a variety of simple patch lubes. The simplest being spit and the next being water with a squirt of dish washing soap or a mix of 50% dish soap and water. Even with these relatively easy to load patches and balls, a short starter will be of benefit to the loading process.