I took out the measuring tape and a scrap of coroplast and mocked up what a partial kamm might look like in the center of my roof. Rough measuring puts the angle around 15 degrees, but it looks a bit steep to me. At this angle, a 14 inch extension goes to the second defroster wire past the stickers, which leaves plenty of area open for the mirror. Maybe a 20" extension with a few degrees less angle?
The window angle is at about 35 degrees, so I'd be reducing drag, but increasing lift. Since I'm rarely over 70 on the freeway, it shouldn't be too much of a problem, especially with the diffuser on the back and the kamm being short.
I did some additional measuring and trimwork, and the angle in the pictures is a little too steep. I cut a scrap of coroplast into a 12.5 degree wedge and took another look. If I get some time, I'll see about making a rough design and see what it looks like.
Thanks to Piwoslaw providing some handy charts
here.
EDIT: After dragging my fiancee outside to hold the strip, I found that I could get away with a 2 foot kamm at a 12.5 degree angle, and it would go as far down as the third wire in the defroster under my stickers. This still leaves enough space to see out the back, but that's as far as I'll go.