Now I see my mistake. The curve isn't an arc. Since I only have the two main date arcs, which do have the same radius, I spaced out the obvious fact that the other curves would have smaller radii.
The arc does hit the two date arcs very near the right place, though. It's just not flat enough in the middle. Thanks for your responses. I'll keep at it. The sunset/sunrise curve really makes the dial much more attractive and interesting. John
