Some people aren't 100% sure of what "offset" is exactly. For those who aren't sure, offset it the measurement from the centerline of the wheel to the back of the mountingface of the wheel.
If a 18x8 wheel has a stock offset of 25, putting a 18x8.5 wheel on the car would require an offset around 31/32 to keep the wheel centered in the same position. You basically add 1/2 the new width to the offset. If you want to move the wheel farther in or closer to the outer lip you subtract or add to that figure. The higher the number, the smaller the outer lip, the smaller the number the wider the outer lip. Unfortunatley, many people assume that the offset stays the same no matter what the width of the wheel and they order the wrong size wheel.
19X10.5 out back and 19X8.5 up front. I don't feel I am qualified to quote my offsets but Vince at Iforged made the calls and they are sweet. I am just waiting on Bridgstone to make the RE01 tire in a 295 or 305 so that I can achive some more traction over the 275.