you know, the only reason you should use spacers is for flushness making. If you cant buy wheels that fit exactly, like most everyone, and your tire sizes vary, then you want to try to get something as close as possible. If your wheels are too big, roll the fender. if your wheels are a half inch or less inside the well, then use spacers. i wouldnt use more than a 20mm to drive on.
dont think about spacers until you see the wheels on. trying to figure what you need before hand is stupid. figure 5 inches from the rear hub, 4 inches from the frint hub, to be pretty close. that means 8" +0 or 7" ~20-30 for the front, 10 +0 or 8 ~20-30 for the back. those offsets are negative, if you didnt figure that out