Hi Chief!
Section B is the angled corner portion and there is nothing to say you “couldn’t” resize it, but that is not supported in the CHBD for that project. (Can’t do 2 sides simultaneously)
If you study the plan framing, you can do exactly what you suggest. I’ll consider a future plan using your suggestion making the corner section smaller, moving the divide lines closer to the 45 corner.
Use the framing as your guide and vary it in any way you wish, then apply the skin and top using your tape measure against the core frame you come up with.