think one reason for the different perspectives occurs because engineers work with theoretical datums all the time. For example, the 0,0 point for fuselage stations on some commercial and military aircraft is an imaginary point well in front of the nose. The wing co-ordinate system on a swept wing is often skewed at an angle with respect to the fuselage axis system. These references are easy to create and use in the software used by today's engineers.
Builders and mechanics don't want to use theoretical datums. They want physical datums and surfaces that can be measured from. So when I build the Apollo, I will surely measure the seatback bulkhead from the horizontal table that I am building on!
There seems to be a lot of interest in recumbent seating. I found these recommendions on a mining site of all places. These seats are for miners that travel in rail carts (sleds?) thru small passages. They are designed for comfort and support in the recumbent position. I would bias the angle between the seatback and seatpan towards the smaller number to help prevent "submarining" in an accident. Yes, the seatpans are measured from horizontal