The comp ratio is swept vs unswept volume. Very simple. It gets much more difficult once the engine is spinng, as now the cam timing, condition of the valves and ring seals and head gasket play a part, along with volumetric efficiency, temperature etc. To figure out an unknown engines comp ratio from a comp test is never going to be accurate. Cam timing plays a huge role. If you are buying a built engine ask for the actual figures. If you are happy to assume its a high comp ratio motor based on that test, (which changed a fair bit once you asked for confirmation) by all means buy it.