I've built a couple of 12's and made nuts for a couple more. My method is slightly crude but it works. First, the caveats - I play finderstyle, thumb fret the "low E" (it isn't really because I down tune) and use different string gauges on different guitars. However they are set basically the same.
Offset on #1 and #12 is 0.105 from the edge of the f/b. The centers of the 1st course is 0.100. The centers of the 6th course is 0.105. I mark the 1st and 11th string, then use the StewMac spacing rule to space the primaries. I then just shift it over from 11 to 12 and mark the octaves. The math doesn't quite work out but its close enough. I also just measured my long scale 12 and even tho I know I did it that way the spacing is slightly off - I don't notice when I play it.
FWIW, I also cut the nut slots slightly less deep on the octaves - more so the tops of the strings are on a plane. I play a lot of slide and the thought is that I'll get less rattle on the lower end of the f/b - in theory however the octaves would be harder to fret.
