How to calculate spacer needs

[ScottD3]

Following my car back from Wales the other day and looking at the wheels, I thought they could do with some spacers.

This got me thinking.
How do you work out what size spacers you want or need?

 

[Jason.s]

You just go for whatever you think you need lol. Dont think theres a set way of working it out. Could try a few things like in this link?

Everything You Ever Wanted To Know About Wheel Spacers - Bimmerforums - The Ultimate BMW Forum