Steve, for a 27 draw I would go for max 58 60 ntn, given a flatbow design and osage. The curves in the drawn bow (side view) should be narrow enough to get a bit of the flattening out effect.
Flat back: sorry, but I have no experience by doing a flat back into a convexe back. I think when you do so, you have to work absolute precisely. I'm curious looking forward what you do.
The ridges should be thin enough to reach a little let go (flattening out).
I see the ideal diameter in a good bowwood like osage at about only 2, with white woods at about 3.
Good luck on this interesting project!