Your statement about not being able to tell anything from your leak down test is correct....
I think you should put 100 psi into the cylinder to start the test... this is still only about 50 percent of working pressure...
The pressure is going to equalize with the outside air pressure at some point... this is why it is called a " leak down" test instead of a " does it leak" test.... so YOU MUST TIME THE LEAKAGE RATE in my opinion to have any useful information.... and unless you can find some information about the speed it should leak... you are really just finding a base line for future leak down tests of your engine...I hope you can find some figures as to what it should be because the leak down test is a good test....