[HawgRyder]

Easiest way I ever found was to screw in a fluid filled burrette into the plug hole (used coloured alcohol) and run the piston through one complete stroke with the rockers not attached.
This makes the fluid fill/empty in the tube and if it's graduated in cc's ... you subtract the little number from the big one to get the swept volume...and thereby the true compression ratio.
Any other method does not take into account various small irregularities (tulip valves/valve head shape/etc) into the math.
Bob