What is enough vacuum for power brakes?

I ran that cam with power brakes and a 4-speed, in my 367, no problem, BUT NOT with the dual diaphragm.
I had to swap in a single diaphragm .

The thing is this;

The booster has a minimum vacuum it likes to see and as long as your engine is creating it and your check-valve is trapping it, the booster will work.
With a 4-speed and downshifting, there was never a time when the vacuum fell too low.
The only time I had problems was when sitting at a light, on an incline for two minutes; the vacuum was not being replenished and I had to continuously increase my foot pressure not to roll backwards. I installed a roll-control.
But eventually, I took that booster apart and not finding anything wrong with it, and having a booster from a 73Dart, I decided to try it. It worked well enough that 20 years later it is still on there.
To charge it up in the morning, I just blip the throttle a couple of times before backing out of the carport.
I swapped out to a 15/16 M/C, so even if I loose a lil assist, it's no big thing. Plus this M/C has excellent moduability?, that is to say; it is easy to modulate.