Engine Vacuum and power brakes ?

The area of the booster diaphragm is also important. Whatever the vacuum amount is, it acts against this area. Some boosters are dual diaphragm, which increases diaphragm area & the force the booster can produce.
I have seen cars with 12" of idle vac have satisfactory booster performance. Bear in mind also when you initially hit the brakes, rpm will be above idle & vac will be higher than idle vac.