I always assemble p&c and head on engine on stand with some grease to seal then tilt engine and fill through spark plug hole with water. Easy and consistent readings. Never had any water going the wrong way. Need to ensure 100% fill obviously and not leave water in there over night. I usually measure 3 times to verify consistent readings

I take this one step further, I do this on a bench then I measure simple piston deck height in several .010" increments, read the cc and do the CR math. When I assemble the engine I then only have to measure simple deck height and look at my notes to get my final shim/CR.

Tip if you use water add a drop of dawn dish soap to break the surface tension, less trapped bubbles.

Looks like you are using reground valves, they sit lower in the head thus increasing the space in the combustion chamber