I am happy to see such thoughtful discussion of the Specs. Richard, I like the direction of the discussion on the seatback, I believe it is capturing the intent of the seatback Spec. Unfortunately I think it is too complicated, and while being precise it can lead to confusion when trying to understand it. I need to give it more thought. As far as the 1/4″ thickness, I think many builders use 2 layers of deck material which makes it 6mm min (0.236in).
Peter makes a good point that these may not be interpretations if it would introduce new measurements to the Spec.