Dear Dirk,
I could search for an answer for you.
Here is some background info on arcs and other curves, for instance:
http://thebuildingcoder.typepad.com/blog/2010/01/curves.html
More comfortable and reliable for both of us might be for you to try the other Arc.Create method instead, taking XYZ, Double, Double, Double, XYZ, and XYZ arguments, specifying center, radius, unit vectors, and angles.
That gives you total control over the definition of the start angle.
I hope this helps.
Cheers,
Jeremy