CLS wheels!?

The CLK wheels have 18x8,5ET30 and I was playing with a wheel size calculator and the difference is small between the two size wheels, but I don't know if it will be rubbing the wheel arch or not!
PS: I have CLK63 OEM wheels, but the rear ones look a bit thin and I want to have something wider at the rear, and I found the CLS ones to be a good match. I'm thinking to change only the rear ones, since I like alot the AMG wheel design.

Smaller the ET more outside will be the wheel correct?
If 28 is more out than 30 why should I need a 10mm spacer?
Thanks








