A Compression tester doesn't test Compression Ratio. The only way to do that is to measure it and that really isn't easy.

A standard RST is over 8.0:1, but needs to be lower than that to run big boost. 7.5:1 or lower. The higher the Compression Ratio, generally the more responsive the engine, but you won't be able to run as much boost before detonation occurs, so its a fine balance. Most people opt for low comp to be safe.

Compression ratio isn't something that changes or is better or worse on a good/bad car. Consider it an 'immovable' factor that only changes when you make a change, like having the Head skimmed if the Head gasket fails etc (this raises the Compression Ratio, which is generally a bad thing for a Turbo engine).

A Compression tester will test for loss of Compression (pressure) due to things like Head Gasket failure/Piston ring problems/Valve seating problems etc.