I'm certainly not an expert on this - I'm still on my first SOF. I'm
using Cunningham's book as well. I did have a thought though: I think
as you increase the angle of the gunwales you'd also be increasing the
height of the bow and stern. (I'm sure there's a correct term for that
but it escapes me now.) If you had no angle to the gunwales then the
deck would be flat from bow to stern. It's the angle of the gunwales
that determines how much "swoop" there is to them.

You might be able to try out some different angles before you cut the
mortises. You'd just have to make spreaders at various angles, clamp
it all together, and see how it looks.