Thanks to all the great wheel/tire threads on FABO, I had a good idea of what size tires I could realistically fit under the car. I ordered a set of Nitto NT555 285/40ZR18's for the rear and 245/45ZR17's for the front. The Street Lynx system allowed a little more room to get a wheel with a deeper offset in the back.
With help from a Tire MountMate we were able to get an accurate reading on what offsets gave proper clearance in the front and back. If you're ordering custom wheels I'd strongly recommend using a tool like this just to ensure you get proper measurements.