Flush wheel offset

[Sylfer]

Hey guys

I drive an f10 520d 2016lci model

I'm planning to buy new wheels

Those come only in
8.5J x20 ET 20 - 40
10J x20 ET 20 - 40

What would be a good offset for a flush setup ?
I want the wheels to be as much outwards as possible but not poking out or rubbing the fenders...

My car is on a KW V1

Front 8.5J ET 20 or play safe and go for ET30 and use a spacer
Rear 10J ET 30 or play safe and go for ET35 and use spacers

Thx for any info