snap on mt324 cylinder leak detector

I never understood the single gauge leak down testers, I have a aircraft dual gauge one but they are all the same. The first gauge is the line pressure 100 psi, if the second gauge reads 95 psi the leak down is 5%, very simple. How do you get a % of leak down with out a reference pressure, I know they work and a lot of company's make them, I just could never figure them out.