Seems this question is being asked every couple weeks now. Here are a few links to help you.
How to calculate your desired offset - http://forums.vwvortex.com/showthread.php?1879943
Tire Size Calculator - http://www.miata.net/garage/tirecalc.html
Offset Calculator - http://www.1010tires.com/WheelOffsetCalculator.asp
Tire Stretch Gallery - http://www.tyrestretch.com/
Fitted Wheel Database - http://www.rimtuck.com/