I found this on the interweb and it kinda makes it easier to understand if you have the original and new rim specs.

http://www.1010tires.com/WheelOffsetCalculator.asp

Liam

Re: [LINK] Offset Calculator

Good find, Liam!

Re: [LINK] Offset Calculator

Mmmmmm useful.

great resources. Thank you for sharing with use
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and if you ever have alloys of unknown PCD then this cheap tool is helpful
http://www.bodsrods.freeservers.com/PCDtool.htm