The inside of the wheel will likely rub with just a 15mm spacer. I have 10s on the rear of my Box with et47 and it's quite tight on the inside (you will need to tie back the e-brake cables, or they'll rub).
By "outer edge", he means the limit of acceptability.
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JGM
2002 Boxster S
1973 911 Green FrankenMeanie
PCA DE Instructor circa '95
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