Thomas, awesome job! Especially that tongue-and-groove.
I've never had the opportunity to build an eyebrow dormer. Some framers use plywood ribs for the arcs. The framing can also be achieved with purlins. The compound angles may be determined by projection or development as you did.
Here are some links to Excel and Javascript "cheat sheets".
Purlin/Square Fascia intersects Main Roof (Excel)
Purlin/Square Fascia intersects Main Roof (Javascript) . For an eyebrow domer the main roof pitch remains fixed while the Dormer pitch is assigned the slope of the curve at the point of interest. (Use MSIE4 or later for the web based calculator. It seems my code gives Firefox indigestion).

Again, I'm impressed by how clean your cuts are on the T&G. In theory the saw blade bevel would constantly have to change as it follows the miter of the arc (the curves would be similar to the sinusoids in the link below):
Development of Circular or Elliptic Dormer intersects Main Roof