OK, Gents. I found this rather authoritative post elsewhere and it seems convincing enough to me.

We've all (or most of us anyway) heard the term "compression testing you motor".

What many people don't realize (as I didn't either for a long time), is that simply going out and buying a compression tester and slapping it in the sparkplug hole may not give you the correct reading you think it might.

There are many things that need to be taken into consideration when compression testing a motor.

One of them is the length of the compression tester tip.
The tester tip should match the length of the sparkplug tip that is used in your motor as recommended by the manufacturer.

Another thing to try to avoid are attachments such as thread adapters that will effectively increase the volume of the head, thereby reducing the pressure read. (the actual pressure in the cylinder will be higher than indicated on the gauge).

Avoid the rubber style "push into the sparkplug hole and then hold it" compression testers. These are inaccurate at best, and often cannot seal the pressure of many high pressure set-ups.

Cheap compression testers (screw in type) will not give you true cylinder pressure readings, HOWEVER, they CAN be used for comparison purposes to determine if rings are getting worn AS LONG AS THE SAME TESTER AND FITTINGS WERE USED WHEN THE MOTOR WAS FRESH, AND READINGS WERE WRITTEN DOWN.

Just adding one adapter can reduce the reading by 20 lbs or more!!

The "best" compression tester to use, is the "Snap-on" line of testers.

They have the schrader valve inside the tip itself, and are very accurate when taking pressure readings.

So next time you hear someone say they have "X" amount of lbs of cylinder pressure, or "your rings are ok if you still have "X" amount of cylinder pressure, take it with a grain of salt.

Because unless they have used the above procedure, OR have spent lots of money on a truly accurate compression tester, chances are they don't REALLY know.
Nor do most of your "average Joe" riders want to buy the expensive set-up to really know what the "actual" cylinder pressure is, or should be."

http://www.allthingsmoto.com/forums/f-114/good-compression-gauge-11506/

Given the number of adapters used and the extra volume added by the tubing itself, I'm quite sure I've lost a few dozen psi that way.

As several of you have pointed it, it's the consistency that matters.

Stick a fork in this 'fread. It's done.

Thanks all!

Edit: further along in the thread I linked to is an even more in depth explanation.