[Walt Fricke]

1) Regulating the air to 100 PSI makes the math easy - the difference between 100 and the reading for the cylinder under testing is the percentage.
2) One of the most important parts of a leakdown test is listening for where the air is leaking out. A rubber tube shoved in an ear and into the exhaust, into the intake, and into the breather will give you a good idea of where the leaks are. If you have a stethoscope, just take the end part off to use it this way. Always a little into the engine past the rings unless you have Total Seal rings. A burnt, bent, or otherwise not sealing valve is pretty obvious, even at fairly low percentages.