Simply put as the volume of the cylinder at BDC divided by the volume of the cylinder at TDC.
For a clean-up job, the compression gain is negligible. My heads only needed about .004"-.005" clean up. The shape of the chamber determines the effect of compression whenever milling the heads. I have cut as much as .170" off of heads (not SC) only to gain 1 point of compression. I had to reshape the combustion chamber, though.