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Man, that set up is confusing. A person can't even tell what the % gauge pressures are. The set up that is used in aviation, just has two identical pressure gauges. We apply 80 PSI to the cylinder being tested with it set at TDC on compression. The average leak down is between 70 and 78 PSI on the differential gauge. The limits for Lycoming engines is no less than 65 PSI with no more than 15 PSI between any separate cylinder reading.
If it's leaking pretty bad then a person can listen at the exhaust or intake to see if a valve is leaking. If it pressures up the engine venting then it's going past the rings. This can be due to aligned ring gaps at times so I always run the engine and check the low one again to see if it improves. If you squirt oil in the cylinder and turn it though, you will have an improved reading if the rings are worn.
With that percent gauge, I would pressure up an air bomb and see what the thing reads at the pressure I use to check it and see if it is accurate. I have my tester gauges calibrated ones a year to make sure they show proper and equal pressures.