Jerry;
How do you figure the necessary boost?
You can get a ballpark guesstimate by figuring the square inch area of the booster diaphragm and converting it into pounds of thrust at a particular manifold vacuum. For instance, just picking an example off the top of my head:
15" of mercury manifold vacuum is about 1/2 an atmosphere of pressure- - - -conservatively, 7 pounds per square inch. The area of a 7" booster diaphragm is:
3.14 X radius squared- - - -38.5 square inches
Booster area times 7 equals 269 pounds of push- - - -the booster makes your leg that much stronger. As the booster diaphragm gets bigger the push on the master cylinder piston increases somewhat geometrically.
12" booster diaphragm = 113 square inches
113 X 7 = 791 lbs. of push on the master cylinder piston. Where brake boosters are concerned, bigger is almost always better!
Jerry
