My stock wheels gave me about a 5/8" radial clearance with my 19" summer wheels. The clearance now with my winter 18" wheels is approximately 5/8" radial clearance (as close as I could measure it).
As you most likely know, to get your actual clearance you can measure the inside bore diameter your stock summer wheel, subtract the inside bore of your winter inside wheel bore and divide by 2. That should give you very close to what reduction of clearance you can expect.
I don't know what a safe clearance would be, I would expect 1/4" would be safe, however, don't take my word for it do check with your friendly BMW service manager or a reliable tire / brake shop.
some more references for you here ... (or google it).
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