Spacers for track car. What's max for rear and front?

What spacing do you run comparative to front and rear? As in if you run 10mm spacers on the front do you run 10mm on the back? If you run a wider track at the back than the front or visa versa what effect does this have on turn in and steering characteristics? Assuming camber is the same front and back?

Normally on FWD the rear is narrower than the front to make it understeer less. But all that changes when you start playing with different geometry settings and suspension anyway. Personally I'd like to go as wide on the back as possible

^^^ 30mm on the front is the maximum. After that it's impossible to stop the rubber contacting the rear arch fixing while in a tight bend. Even with these had to roll back the arch liners to the metal