shouldnt we be able to determine what the comp ratio is by the psi on the compression tester

 

No.

There are a number of reasons... leakage, different cam profiles that cause a variation in how much compression is actually captured, the fact that compression is more or less an adiabatic process except that there's variables in how much heat escapes during the process, etc. etc. etc.

 

No but the test result from a cranking compression test can be used as a "guideline" to indicate what the motor will need for fuel octane.
180ish PSI is usable on pump gas.
over 200psi gets a little too dicey.

 

I'm alright on pump 93. 210psi 10.25:1 CR Aluminum heads though.

 

130 psi is okay on a small block chevy with 64 cc heads and flattops? it's a 350 cu.in with standard normal deck heigh.
thanks

 

Depends on what cam is in it.

If all 8 are the same within 10 psi or so, and the motor runs right, then yes.

If of course, the test was performed properly.

A motor that actually had that little compression would have trouble running, unless this is an all-out race piece. Which is why I feel that it's necessary to add that little qualifier.