Well, honestly I'm not baffled, just seems odd you would imply there is no way possible to reference deck height or squareness when you obviously have the knowledge and awareness of how to do this stuff correctly.
A simple process of indicating in the plate and a measurement to it does both of these things.
Yes, I agree and am aware most shops that make claims dont have the skills or equipement to do this. My block fixture was the first thing I made when I bought my surfacer.