you guys are making this harder than it is. I suck at/hate math and I have no problems figuring out which combos will fit my vehicles/customers vehicles.
a) there are TONS of awesome "will it fit" calculators out there. As long as you know the stock tire/wheel size and offset from your car, the rest is gravy.
b) if you want a general idea of what will fit, just do some really simple math and take a look at how things sit now. Finn, if your stock wheel is 6.5" wide and a 48mm offset:
you can subtract the stock rim width from the new rim width and divide it in half - in this case the added wheel width will move the rim .5" inches closer to the fender lip. Do the same for the offset - 48mm-38mm for another 10mm closer to the fender lip, or another .39". Measure out .89" from the tire/wheel edge of your existing setup, and you can ballpark whether it will work or not.
Of course there are many other factors (camber, drop, tire brand and width, etc.), but that should get you started.