The first thing is, "leak down testers vary." That is there is no hard and fast calibration
Basically what you have there is an orifice of some .040" with a regulator supplying whatever the destructions say, usually 100PSI "blocked"
When you hook up to the cylinder, the orifice restricts flow, and if there is a large "hole" in the cylinder to flow through, the more flow, the more drop
Ideally you should run this warm, but that can be difficult to do.
Also, without a "before" baseline test, all you can do is to make assumptions about the condition of the engine, and compare one reading to the next
As you are doing so, listen at carb, exhaust, oil filler, and rad filler for an idea of leakage.
About 15% leakage or so is about getting bottom, depending. 20% is "you have trouble"
If you have a 0-100 gauge on the orifice side, this equates to the reciprocal of leakage, IE 80 psi is 20%, 90 is 10 % leakage
Mark the dampener at 90 intervals for a V8 so you can wrench the piston being tested to exact TDC. Otherwise, the air will just push it over.
If you can, best to ALSO test with the piston at BDC, I always do the TDC tests, then yank the valve gear off so the valves stay closed, and then just let the air push the piston down to BDC and re-reun the test.
Hope this helps