you know the stroke, the CC of pistons, CC combustion chamber, headgasket thickness and how far under/over the piston sits compared to the block deck. From this you can work out an accurate compression ratio.
Just by going from the guage u have a few problems, different guages and cam overlap. You have static compression ratio, and dynamic comp ratio, so when you use a comp tester the cams if they are larger or set to a different position that can bleed off more/less air, therefore changing your readings.
also, you need to go thicker head gasket when you go oversized pistons (if you want to stick to the same comp ratio), unless they dish in the piston is larger as the comp ratio will rise.