To work with the actual braking ability you really need to find what is called the effective radius of your brake rotor. To do that you need to know the caliper piston diameter (only the biggest one for calipers with more than one piston) and the diameter of your rotor. Effective radius is the total diameter of the brake rotor plus that same diameter AFTER you subtract the diameter of the piston. Then, divide by 4. Use a "big" Mopar 11.75" rotor and the 2.75" caliper we used above. 11.75" minus the 2.75" piston is 9 inches. So, 11.75" plus 9" is 20.75". Divide by 4 and you get a 5.18" effective radius.
Effective radius of the rotor is unrelated to the piston diameter. It is the center of contact for for the brake pad to the rotor. Typical undergrad engineering calculation put it outboard of the center, but the taper wear of pads typically moves back inboard. So the center of contact is good compromise.
