Top Hat - Not a direct answer to your question, but
http://autospeed.drive.com.au/cms/A_108913/article.html has a discussion about rings and compression. An extract (deals with motor vehicles but it reads across to aero engines in basic principles)
During operation of an engine, blowby passing through the ring gap is under very high pressure and it reaches what is termed "sonic velocity". At this point any increase will not result in any increase in gas velocity (hence the rate of escape of the gases). From this point of view, leak-down tests are of little or no value in determining engine condition, as they measure leakage under static and low pressure conditions.
The gas which does escape the piston rings, and which is returned to the intake charge, is termed blowby. The control of blowby to an acceptable level is important because excessive blowby means loss of engine power, leads to ring sticking and an increase in emissions. That is why manufacturers tightly control piston ring shape and flatness, two factors important for good gas sealing. Blowby is measured under laboratory conditions by sealing off the engine's crankcase and attaching an accurate gas meter by means to the breather on the oil filler cap. A typical 2-litre engine can expected to have a blowby figure of 20-30 litres per minute under full load conditions.