I know this is a long-dead thread resurrection, but I came across a couple of great posts about drawing eyebrow dormer geometry on Sim Ayers' "Roof Framing Geometry" blog, which has all kinds of great stuff. Here are the links:

http://sbebuilders.blogspot.ca/2014/10/fledermausgaube-mit-gerader-kehle.html
http://sbebuilders.blogspot.ca/2014/12/the-eyebrow-roof-dormer-with-straight.html

Both of them describe the process for an eyebrow dormer with a straight valley in order to have greater flexibility with roofing materials, at no great loss to the overall appearance in my opinion.