Measuring gasket volume

How do you go about measuring the gasket volume under compression, how do you achieve this compressed state while also allowing room to measure?

I think you just use the "compressed thickness" value from the gasket manufacturer. They have done the engineering / design to make the gasket compress to a known thickness.

If that is unavailable, use the thickness of the fire ring of the old gasket, if you still have it - there will be slight spring back, but very little, especially if it was overheated.

If I have the decimal point correct, for an 86mm bore a 0.001"/0.025mm variation is equal to 0.145cc, so a slight variation on the actual Vs estimated thickness may not be that critical for calculating CR. Even for piston to head, piston to valve, or quench, it shouldn't make that much difference, unless you're running things extremely close to the limits.