So in other words, if I'm running -3º camber in the front, I'd set the rear at -2.25º. (i.e. 3 subtract 0.75 equals 2.25)

Or if I were to run -2º in the front, I'd set the rear to -1.25º

You want a little less static negative camber in the rear, because the rear gains negative camber in compression at a higher rate than the front (due to the suspension geometry).

So in other words, if you start with a static camber of -3º front / -2.25º rear, as your suspension compresses in a hard turn, the two different camber rates will converge into equal values, giving you a balanced car.

Hope that makes sense.