When I first tried to model my intended roof, to check for the desired overall shape, I was assuming that panels would be straight-sided isosceles triangles. I obviously wasn't thinking straight. Straight sides would only be the case if they formed a straight-tapered roof.
Whereas, bi-pitched roof panels are bent at the junction between the upper triangle and the lower trapezium. The width of this joint must be greater to allow for the depth of the bent form. [Spherical geometry?] This width should, of course, be 2.77 units. Exactly as calculated above. Fingers firmly crossed as I take scissors to cereal packet cardboard in the time-honoured way.
Whoopee! Almost unbelievably, the model appeared exactly as I was expecting! Next time I'll add double tabs to my four sided units so that I can use clothes pegs on the triangle's inside joints. I'm waiting for the PVA glue to dry on three triangular panel tabs so I can finish the model and take some photos as proof of my calculations.