To measure the thickness you need a micrometer or calipers. Its been a long time since I've purchased pillow ticking, but the faberic shop may be able to tell you. If you purchase precut patches, the thicknes will be marked on the package.
You should have a snug fit, but not have to beat it down the barrel. I would purchase one small bag of the .015 and one of the .010 and go from there.
Take about five shots and try to find the patches. They will be anywhere from about ten feet to twenty feet away if the wind is not blowing. Look at the black around the patch. It should not have black all the way up, you should see a definate line where the powder did not burn it. If the whole patch looks black, then you have "blow by" You do not want that because is indicates the ball is too loose. The second thingto look for is the patch being cut up. That is an indication that the ball is too tight.
If you have trouble reading the patches and can take pictures and someone here can tell you.