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Doc GTO summed it up nicely. 12/20 or 15/25 are the most common fitment when using OEM BMW wheels.
When folks move to a different wheel/offset, usually that wheel has the proper offset for the look you are going for. If not, you can possibly use a spacer and adjust accordingly.
There is no math or formula that would apply to tell you how much more wear you may get on a wheel bearing by going to a higher offset or larger spacer. It's the trade off you take when modifying your car.
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