Here's the thing about leakdown testers, they really are not measuring percentages. They are measuring the pressure drop across a small orifice. As the flow goes up the pressure drop increases also. And as Thunder pointed out this is under static conditions. The percentage number is strictly relative to that particular flow meter.

They are great for diagnostic use to isolate a bad cylinder, or to get a baseline for a particular engine. But don't fixate on those numbers! 12% on the leakdown tester does not mean you are losing 12% of the combustion pressure.

The real test is to measure the actual flow out the breathers on a running engine. Back in the day we used a J-tec flow meter on our dyno to do just that. I cannot remember what the numbers were but they were really low on a healthy engine. I think I'll look for some of our old dyno sheets to refresh my memory.

If you can feel puffing or a breeze out the breathers there is a problem. A happy engine will just have a little vapor drifting out.

+1