I do all my measurements to under the cavplate about 60mm apart to get my height and angle. C is just a reference (at the drive dog) for anyone using a different lower unit...which would be a different setting for that other lower unit anyway as they all need different values to make the boat run similar.
(Just looked at the sheet again and i think i see your question) The C value listed is not the angle...it is a height to center line of shaft at drive dog.
The trim angle is about 1.5neg