But, what IS "the right thickness?"
There's a rule of thumb which would indicate that the diameter of the gas check shank, minus the sized bullet diameter, divided by two would yield the optimum GC thickness, but it's really more a starting point. It MAY work fine for you, but then we have those pesky tapered GC shanks, oddball moulds, hardness of the GC, softness of the bullet, etc. which all seem to bear on making the calculated optimal "wrong." Hence - experimentation.
I don't know if this helps at all, but through experimentation*, I found that certain thickness fit MY GC shanks well. Turns out that the right thickness for mine wasn't necessarily the right thickness for others' - or the usual optimum thickness for a given caliber.
*Through the luxury of access to the gas-check-maker maker, Charlie, and a variety of samples and significant patience from Gil, I was very fortunate (spoiled).
.0095" for my 35s, mostly sized to .358" or .359" (rifle/revolver)
.008" for my 22s, all sized to .225" (rifle)
.014" for my 30's, all sized to .310" (rifle)
Ideally, one would find a cheap/free and reliable source of raw material soft enough to not induce an added step of annealing, but we can fudge on that last one depending on how much time we can spare in lieu of cash.
Time? Well, we have to be able to practice the fine art of denial when it comes to time. I've tried to explain to people that it takes me less time per shot to load my muzzle-loader (patched round ball and powder - no "pellets" or pre-charges) than it does for my 223. Of course, no one ever "gets" that one and no one ever "bites." I get the faux-knowing-nod and a wink, like it's an inside joke.
If we have time to cast, we have time to make gas checks. I set my stuff up and crunch out a hundred, get bored and quit. But, I leave it all set up and stop to crank out ten, twenty or fifty between other tasks or while thinking about how to do what next on a project. After a few days or a week of intermittent "punching," I have quite a pile of gas checks. I like to fill little glass spice jars with them to set on my shelf so I can look at them and feel wealthy and fortunate. Seriously. Just as gratifying as looking at a stack of cordwood, which always induces a sense of calm well-being.
How many gas checks does one need?
It may be as well to buy a thousand and get it over with. I bought enough coil stock for each of my GC makers to last longer than I would, and that's not a lot of coil stock. Three to six one-hundred-foot rolls of a given size goes a long, long way, making tens of thousands of gas checks. I can get about 3,000 30 caliber GCs from a 100' roll. I think I was paying about $8 per roll with shipping. That's about $2.67 per 1K for material. Even being a bit extravagant by buying coil stock, I am getting off very cheaply.
I have an old friend who is finally gearing up to cast afte several years of me nagging. Today, he told me he was starting with plan-base moulds only and no gas checks. "Well, well, Mr. Confidence, what make YOU think that you will get away with nothing but plain base right out of the gate?" His logic was something I hadn't considered, but sound. Figures he'll only use GC bullets for hunting and a few hundred will last a long time. He can buy that many or get me to make them. No sense spending money on the tooling that would get little use. Oh, don't worry, I can get him to do stuff for me cheaper than I can do it myself too. This guy's an amazing knife-maker.
Geez, sorry that was so long again.