Pistons have a published compression height. If you know your true deck height you can then calculate how far down in the hole you are.
My engine is milled to "0" deck, and a 440 w/ flat tops. So, in my case I am 0.0" down in the hole. It also means in the quench area the clearance from the top of the piston to the head (assuming negligible rocking) is the thickness of the torqued gasket (another published number you will need).
Actual Bore Area times Stroke plus how far your piston is down in the hole x bore plus gasket diameter x gasket thickness plus your chamber size minus your piston dome is your starting volume at bottom dead center. Let's call this "A".
How far your piston is down in the hole x bore plus gasket diameter x gasket thickness plus your chamber size minus your piston dome is your end volume at top dead center. Let's call this "B".
A divided by B is your static compression ratio.
When someone says a piston is advertised at "12:1" they are making assumptions about your engine and chamber. Usually they assume you are "0" deck. I have never seen a factory 440 that was true zero deck when I checked it, usually closer to 0.050" down or more.
My 440 is 0.030" over, stock 3.75" stroke. Engine is "0" deck. Fel Pro blue 0.039" gasket w/ a 4.41" bore. -3.5cc dome Ross pistons. As measured 75cc chamber. This yields a static compression of 11.3:1 with my "10:1" pistons.
Coincidentally, if I had 88cc chambers it would calculate to be 9.999:1 compression (close enough to call 10:1).
Hemis are a whole different animal, huge dome and monstrous chambers... but the math is the same.
