Proper way to adjust proportioning valve?

The best way to adjust it is to obtain the help of another person and head to a deserted good sized parking lot. Fuel load should be about 1/2 tank. Have the other person watch from outside the car -- both sides -- while you make stops from 45-50 mph braking hard enough to lock the tires. Your 'spectator' will tell you which end is locking up. Keep adjusting the proportioning valve until the front brakes lock just a hair before the rear brakes do. The front brakes do MUCH more of the stopping in a car like the Stang when the proportioning valve is adjusted properly. You don't want the rears locking first for 2 reasons -- 1) when the rears lock first you're not getting maximum stopping power out of the fronts, and 2) the rear end tends to come around.

Yup - you really can't do it correctly by yourself. The beauty of ABS -- it takes care of proportioning on a wheel by wheel, stop by stop basis.