What is enough vacuum for power brakes?

Here's a test for you;
With the engine off, and a vacuum gauge plumbed to where you can see it from the driver's seat;
Pump your booster down, to less than 7 or 8 inches.
Next, apply the brake pedal with modest pressure; then start the engine and let it idle, while watching the vacuum gauge.
IF/when the pedal starts falling, check your gauge. If the pedal does not fall, then it is not being evacuated, so rev the engine up an inch or two and wait. It only takes 5 to maybe 10 or so seconds to evacuate the chamber. Continue in this way until the pedal drops.; then immediately check your vacuum gauge, and your rpm.

What you see on the gauge is the minimum vacuum required to operate this particular booster, and the rpm is the minimum rpm to get the minimum vacuum. Write this down.
If you have a dual diaphragm booster I can almost promise you the minimum vacuum will be up around 12 inches.
If you have a bigger single diaphragm, probably 9/10ish
Now, with your foot still on the brake,read your vacuum and shut the engine off while not moving your brake-pedal. How long does it take before the vacuum falls to the minimum? If it drops like a rock, you have a bad booster, abort the test. But if it holds for more than one typical braking application of say 10 or 15 seconds, then you are good to go.
If you aborted the test; restart the engine and get the booster charged. This time remove your foot from the pedal, read the gauge, then shut the engine off, while watching the vacuum gauge. Ideally, your vacuum gauge reading should hold for hours. But if it drops like a rock again, then either; the checkvalve is faulty, or the diaphragm is ruptured, or your control valve is leaking, or your pushrod is too long. The rate of loss can point to the faulty member, but since we have no baseline, it is as good as impossible to use for this test.
Usually if the diaphragm is bad or the C-valve is faulty, then you will hear the hissing up under the dash.
To check the checkvalve, you can have a helper pinch the line at high vacuum, and watch the gauge, as you shut off the engine. But this only works if it is the only faulty part of the system.
Pushrod length is harder to set.
The length has to be synchronized to your driving style.
If it is only a little too long, then the booster will give instant assist, which is extremely annoying if you tend to brake late.
If it is only a little too short, then boost is late, and it takes more pedal to get assist. and you get to thinking it might not be working so good.
If it is a lot too long, then you could shatter your control valve on a panic-stop.
If it is a lot too short, boost is very late and quite whimpy.
Once you get it into the sweetspot, 1 turn longer or shorter is a lot. Getting to the sweetspot is a lil harder. There is a procedure in the FSM to get you started.

If you swap M/C bores, you will need to resynchronize for the change in pedal stroke that will occur.