What's the maximum you can get away with on OE wheel and standard 205/45 rubber with no arch mods?

Also I got some wheels that are ET42 (were advertised as ET38) will they go on Fabia VRS with 312s without any spacers until I get round to fitting some? (they are the OE TT brakes)

Cheers.

I ran 20m at the rear and 15mm up front, thought it sat about right. I bought them for running 17's, with the car lowered on coilovers and only then it scuffed the arch the odd time, but that was as low as it would go, with the adjusters out on 17's. Wouldnt go any wider than 20mm at the back, and 15 up the front tbh. You have pm!