If you angle the head (any head, not just the hemi heads)so one side is higher than the other and place the plate so the hole is on the high side next to the edge of the combustion chamber it makes a lot easier to fill the combustion chamber,
Liquid flows down hill in other words
Expereince is a great teacher
I bought a needle tip for a hand held grease gun and use that to place a very thin line of grease around the chambers to seal the fill plate to
BTW, what type of liquid are you using for the measuring? How many CC difference did you find between the four combustion chambers? Or did you check all four, if not you should