One thing you'll need here is how the tire fits on rims of different widths, and you can get that from tire manufacturers. I believe companies like BFG actually publish that info in their catalogs. Pick a tire size, pick a rim width to fit (from the tire manufacturer's recommended rim-width range), then you can do all the offset calcs. An easy-ish way would be to measure the width of the car from the left wheel well lip to the right wheel well lip, divide by 2, add whatever excess you want for the tire to stick out, subtract half the outer tire width (as it would be when mounted on the rim), then subtract half the hub-to-hub distance of the rear end. That should be your offset. Batteries not included, has been shown to cause cancer in laboratory rats, do not take with MAOI inhibitors, may cause drowsiness, and all other disclaimers apply
Cheers,
-Neil.