E92/E93 wheel offsets help.

I have been looking at upgrading the wheels on my E93 cabriolet to 20" rims.

I have been looking at the standard rims and they seem to have different offsets from front to rear and I would really likr some expert advice on the correct set up for the car as I intend to lower the car on KW coilovers and do not want the wheels rubbing the fenders.

Standard set up

Front - 17J8 +37mm offset with 225/45/17 tyres
Rear - 17J8.5 +34mm offset with 255/45/17 tyres

or

Front - 18J8 +37mm offset with 225/40/18 tyres
Rear - 17J8.5 +34mm offset with 255/35/18 tyres

I would like to chamge to a 20"x8.5 rim with a 245/35/20 tyre?

What offset will I need?

I just ordered a set of Beyern Type Mesh Wheels 19x8.5 front and 19x9.5 in rear both with a 30 offset if this is wrong some one please chim in the guy said it wouldnt have any problems and it would look great.