20mm on 305s will be extremely close to flush. You haven't mentioned what wheels these are going on, so I'm assuming it's the sport stockers.
If you drop the car and run a tiny bit of extra camber in the rear, you will need 25mm to bring it out.
here's mine on 305's w/25mm spacers in the rear on swift springs. stock sport wheels. There's a little sidewall bulge with the Hankook ventus v12s im running- if you ran a different tire with a stiffer sidewall such as the stock bridgestones, it would be perfectly flush.
