Here is a simple method for a 66cc motor... thread a plug into the head and make sure it is level. Say you want a compression ratio of 10:1, take 6.6 cc of water, and put it into the combustion chamber. Mark the level at the edge. This will be slightly low to surface tension resulting in a higher cr, though likely less than .1. Note this does not take into account the volume of the head gasket, which can be found by (17.35xgasket height in cm). If you subtract this volume from the 6.6cc, you can use the resulting volume as the volume of water to use in the combustion chamber. Also be aware that the cr is not static, but dynamic in a two stroke due to the open ports and effects of an expansion chamber. There is also a factor of stretch due to the elasticity of the rod and pins.