Front/rear camber "balance" doesn't exist - at least as far as I understand. Front/rear camber differential also has nothing to do with tire sizes.
I know I'm being obvious here, but I have to point out that the rear and front tires perform different tasks. The rears have to put the power down as well as possible and provide the best possible contact patch. The rears also don't turn ('doh) and thus there camber range both static and dynamic vary much less throughout the suspension travel, compared to the front.
The fronts don't have to provide the best possible traction in a straight line, they have to provide the best traction and responsiveness in corners - i.e. when the car is loaded asymmetrically and the center of balance is shifting to the outsides.
Long story short, you need different camber settings front->back since the tires perform different tasks.
I'm currently running -3 / -2 btw. with square 255 tires for racing and 235/255 for the street all without problems.
And just for fun, here's a random F1 picture showing the camber differental in F1 cars:
