[strictlypersonl]

I'm not using an equation. I'm using logic and physics.

I'm using the area of the caliper piston because that is what the fluid pressure is acting on. The pad transfers the piston force (area x pressure) to the rotor, regardless of the pad area. The only reason that a larger pad is better than a smaller one is that the heat generated per unit area is less, so the pad stays cooler.