Usually use the pressure tester to look for a large discrepancy in pressure from one or more cylinders to the rest. If you have 240 psi in six cylinders and 180 on the remaining two there is definitely a problem.
Compression ratio is a different matter, if you have two engines exactly the same except one has dished pistons and one has flat top pistons, the flat top has a higher compression ratio and will have more psi on a compression test. Now if you change the camshaft on the higher compression ratio engine, put in a camshaft that closes the intake valves later than the original camshaft you will lower dynamic compression ratio and the cylinder pressure in that engine. It will still have the same static compression ratio but will have less cylinder pressure than it had with the original camshaft. Now take two engines that are exactly the same, engine A and engine B, they have the same compression ratio and the same cylinder pressure. Now change the camshaft in engine A. The new camshaft for engine A closes the intake valves sooner than the camshaft in engin B. They both have the same static compression ratio but now the valves in engine A closes the valves sooner than the valves in engine B. Engine A now has a higher dynamic compression ratio and a higher cylinder pressure, but they both have the same compression ratio rating.
A leakdown tester or cylinder differential pressure tester will put air into the cylinder at a specific psi, 'used to use 140 psi on piston engine propeller equipped aircraft. One gauge of the tester tells you psi of the air going into the cylinder, the other guage tells you the pressure the cylinder is maintaining and thus the pressure loss. If there is a large pressure differential in the cylinder it can indicate a problem in the cylinder such as burned valves or worn rings/cylinder walls. Listening for and feeling air escaping out the crankcase breather MAY [MAY] indicate a ring or cylinder wall problem, air leaking out the carb would indicate an intake valve leakage and air out the exhaust would indicate a leakage past the exhaust valves
