How do you gauge the accuracy of a leakdown tester? What makes one more accurate than another?
You have some latitude on the actual results displayed--My Snap-On seems to be unusually sensitive in that it always displays about three times the expected leakage--nothing I test but motorcycles shows less than 5% leakage; most stuff is 20%--40%. Most Iron Duke 2.5L four-poppers have at least one cylinder at 80%. And that sensitivity is not a problem as long as you're used to the typical results your tool displays.
The problem would be with consistancy: If the same cylinder shows different amounts of leakage on each re-test, the leakdown tester is pretty much junk.
It's not just the gauge(s) that need to be well-designed and manufactured; the regulator must hold a constant pressure even with varying input pressures.
Now, if you have a good gauge or two; and a good regulator--what else is there except some hose and an orifice? Hopefully the orifice will be brass or stainless steel so it won't rust over time from water in the input air, and clearly the hoses have to be reasonably durable and seal properly to the spark plug hole.