This is the formula I had for stopping force.
Force in lbs/ft = pedal force in lbs X system force ratio X coefficient of friction between pad and rotor X effective disk radius (ft) /tyre rolling radius (ft).
Pedal force is how hard your leg pushes against the pedal in pounds.
The system force ratio is the result of combining the mechanical force multiplicaton of your brake pedal lever (simple lever calculation) with the hydraulic force multiplication derived from comparing master cylinder piston area to caliper cylinder area. If power brakes are involved, you have to factor that in also. You will note that this still gives you pounds delivered to the calipers.
The coefficient of friction is also a dimensionless number and depends only on the properties of the iron disk and the pad material.
The terms for effective disk radius and tyre rolling radius account for the leverage effects. You can see from them a good feature of low aspect ratio tyres - you can get the disk radius to be closer to the tyre radius, which will make the brakes work better.
Copied from here, because I could find my origional stuff with this formula in.
http://www.rdrop.com/users/larry/download/Brake_Power_vs_Weight.PDF