has anyone installed 18mm spacer on a stock 997.2 c4s wheels. i know the most common setup for spacers are 7mm and 15mm. It seems to me that 18mm might be a better fitment but i don't know if there is any issue on rubbing. I'd appreciate some help

 

It is borderline too much. Consider that the tire will be bulging slightly out of the fender.
I think 15 is the best number to use...