Welcome to the sport, Dutchman: Your question, what is the idea thickness for a patch is totally dependent on every single gun you will shoot.
I recommend you spend $15.00 and buy Dutch Schoultz System,
http://www.blackpowderrifleaccuracy.com/
to learn how to " Read " your patches. You are looking for a patch and lube combination that will give you a spent patch with brown edges, a dark brown or black ring where the patch actually seals the bore around the ball, and a yellow to white center where your lube is sufficient enough to keep the gases from burning the patch.
You will need to use a caliper or micrometer to actually measure the land to land( bore) diameter of your barrel, not simply trusting whatever the maker puts on the barrel, and also the groove diameter. The depth of the groove suggests a starting point for the choice of thickness of your patch material, ONLY after you choose a ball diameter to shoot.
For most bores in rifled barrels, you want a ball diameter that is .010" smaller than the bore diameter. But different guns will shoot better or worse with different balls. For instance, I shoot a .50 caliber rifle. The Green Mountain barrel has a bore diameter of .500-.501" I can't seem to get a good measurement closer than that. It doesn't matter. I have tried both .490" and .495" balls in the gun, and I seem to get my best accuracy shooting the .490" ball. I could try .485, or .500, or even .505", but am satisfied with the .490" ball. I have been using .015" thick patches, but am trying some .017" and .018" thick patching just to see if I can get any improvement on accuracy. And, although I use a chronograph when I am testing loads, now, I also ALWAYS read my patches.
Best regards.