The compression ratio is determined by cylinder combustion chamber size (they vary with manufacturing tolerances between heads even with the same casting numbers) head gasket thickness and diameter, piston dome/valve relief size and the height of the cylinder block (again this varies with manufacturing tolerances, some are taller than others even with the same casting numbers). The best way to figure ratio is to measure all of the different dimensions of the parts involved.
More importantly is the question "what is my cranking compression with a compression tester?"
A low compression ratio engine with a small duration camshaft will have the same compression as a high compression ratio engine will have with a long duration camshaft. If the camshaft closes the intake valve early (small duration cam) the compression cycle will start sooner and build more compression. If the intake valve closes later (long duration cam) the compression cycle starts later and will not build as much compression. Matching the camshaft and compression ratio so you have around 180 PSI of cranking compression will give you an efficient engine for use with premium pump fuel. Keep it down to 160 PSI for regular pump fuel.
Hope this helps someone!
Sincerely
Dennis Moore