An engine is an air pump and performance is directly linked to the ability to pump air efficiently.
Air is squishy and subject to the laws of physics. Absolute values can be misleading therefore interpreting the results is just as important as having a good test setup and procedure.
Here's some good reading on vacuum, compression and leakdown testing. (Mike: you might like the clever voltage drop method of compression testing!
ASA Tech to Tech
I use a nitrogen cylinder to maintain stable, dry input "air". 100 psi input over 90 psi cylinder pressure is considered 10% leakage. While the value is relative to the laws and argueably subjective it is much better than scratching your head and wondering if the cylinder seals or not!
Listening for cylinder leaks will tell you where the air is being lost. This is very valuable test to do right after the heads are installed on the engine stand.
You don't even have to install the valvetrain. This will pay for itself the first time you find a problem. Much easier to fix at this point than after it's in the car.