850 T-Gul

I'm looking to run 245/40/17 continentals in the rear on the stock avus rims and was wondering what size spacers I should use back there to clear the rear suspension and line up nicley with fender but not stick out too much. 5mm, 10, 15? BTW I know 245 are a bit wide for the stock rim, but the continentals are not that wide of a tire compared to say an S-03 so I don't think it should be that much of an issue. I would just get 235/40 avs sport or S-03s but I only need 2 tires as the fronts were replaced a few months ago and want to have 4 tires of the same brand, and I think 235 continentals wouldn't be that much wider than the fronts and it would be silly to have the rears only 10mm wider than the fronts.Thanks.