While camber needs are somewhat determined by the tire choice (think R888 v RA1) it is more determined by how much the car rolls in a corner (spring, sway, center of gravity).
It is also effected by the geometry of the car. With 911/930's for example the 930 has a more aggressive camber curve because of the changed inner mount. Another example is raising the spindle on the front struts on a lowered car. This restores the front end to a more beneficial area of the camber curve. On a stock car those two things can be good for about another .5 deg of effective camber.
A stock car rolls about 4 deg. With this it looses about 3.2 deg of camber. A car with upgraded sways and aggressive torsion bars might sway about 2.5 deg and lose about 2 deg of camber.
I do not know what the best tire angle is but it is probably something between square to -1 deg to work its best.
Thus a stock car would need about -3.2 to -4.2 to get the best performance out of a tire. For most of this it more comes down to living with what we have and mostly using the outside of the tires on the track, especially in the front where we can only get about -1 deg of camber.
A torsion bar race car might be able to use something between -2 to -3.5 deg of camber. (This fits the data points we are getting.)
A race car with 400/600 springs might reduce neg camber needs by another deg or so.
At least that is what I belive from what I know so far.
