I should have said "With a straight edge across the top of the cylinder" above. Also, John has made his Plexi plate round to fit into the counter bore of the head. A large flat plate across the face of the head will include counter bore volume as part of the combustion chamber volume. You can measure the depth of the counter bore and subtract it out, but it is easier to make a round plate and measure directly for good repeatability. Tim's method does this all automatically, but I suspect he is a major investor in Playdough.