Dimman, I'm picking up what you're putting down. Wheel and tire fitment is almost a game. It really depends on how much work you want to do, what you're going to do with the car and how you want it to look. It also depends on your ride height, suspension travel and alignment as to how much you can tuck.
There is really no way to calculate what size wheel and tire you can fit under there. There are tools you can use to estimate how it's going to fit. Here is a good spreadsheet calculator to compare wheel and tire sizes.
http://members.cox.net/batdog/misc/offset_calculatorSRNH.xls
From that, you can figure out how each setup relates to the other. but now you need to know what will fit the car. You need to look at how much suspension travel you have and what you're alignment and ride height are. Also, if you're willing to do any fender rolling or pulling. A lot of people seem scared of this, but it's not a big deal and is easy to do.
You also have to decide what you want the car to look like. Do you like a little bit of a stretched tire, with a little bit of the tire tucking under the fender and wheels that are flush. If so, you're going to have to fit things differently. If you're running lowering springs on a stock weight car and want to put as much tire under their as possible, you're going to do it differently. If you're building a track car with a stiff suspension and want to put 315s in the back and 295s in the front, then you probably wouldn't be asking these questions.
The best bet is to look at how other people fit wheels. What sizes they're running, how they look and what the rest of their setup is. This will give you the best idea of how things will fit. And then you can go from there.
You could try and measure whatever setup you have now and how much room you have and that will also give you an idea of what you can fit. Use the spreadsheet I posted earlier and mess with the numbers.
So prioritize and decide what is more important to you. Do you want to fit a 285? Do you want flush wheels? Do you want to have more ground clearance than a Hummer? And then after you answer those questions you should have a better idea on how to go about this.
Tim
