I know it may not be the fastest method, but I normally start small and work my way up with regards to thickness. I'll usually rely on my customer's feedback and I'll also go out and watch them skate to figure out if we need to add more height. As far as how tall you can make it, I've gone all the way up 1/4" for a shim for pronation and supination and recently just shy of 1/2" lift for a customer who had one leg shorter than the other. I'll admit I was a little skeptical whether or not I would be able to do 1/2" lift, but I used a few extra coppers to really anchor everything down and haven't had any problems.
If they don't have anything thicker than the 1/16" plastic, they can always stack them on top of one another. That being said, Graf makes 1/4" plastic sheets specifically for shimming.
If you'd like, I can attach pictures of the thick shims and what it looks like on a skate.