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Jim,
You are right about needing calibration. I think this is what you meant, but for the other readers what needs calibration is the tool, not the engine being tested. The size of the orifice determines how much air flow causes how much pressure drop, and what you really want to measure is air flow (i.e. the amount of leakage). Some manufacturers of the testers give at least some guidance on what is a good engine and what is bad in terms of pressure difference. Unlike compression testing, absolute numbers matter with leakdown. That is, you can diagnose an engine where all of the cylinders have some ring leakage, given some gauge calibration (which might include pressure deltas from another, known to be sound engine using the same tester).
On rereading your original post I see you mentioned listening for the leak point, which I missed the first time through. Sorry.
Although neither of us mentioned it on the first time through (I think-), a leakdown test is normally (and most easily) done at TDC on the compression stroke. The valves are guaranteed to be closed there, and if you are exactly on TDC (or at least very close) the engine won't try to turn when the air pressure is applied. As I noted (without starting at the beginning-), if you want to do a leakdown test anywhere other than TDC you need to in some way anchor the crank. Of course you need to pick points where both valves are closed (or in some way disable the valve opening, as you noted). BTW, 100 PSI on a several inch diameter piston makes a lot of force to anchor. It is hard (or impossible) to do it by hand-holding a breaker bar on the crank nut, if I recollect.
Frank
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