Im done trying to beat the dead horse. You my friend do not have a clue about getting a compression ratio and you should find a new mechanic if he is telling you that there is a guage that gives you compression and not just cranking pressure.
Get any manual and look it up, or ask any full time mechanic, not a weekend backyard mechanic.
I guess real world wrench turning experience and ASE certifications take a back seat to the brilliance you can attain by reading a Haynes manual :roll: