Hi Martin, so the "tan" and "perp" snaps that you are seeing are linebased snaps, when you see them pop up they are showing you a tangent location on a line coming from the previous picked point which in your case here is the second picked point of the arc, not showing you the type of tangency that you're looking for which is more of an "full object" tangency.
That type of tangency is usually created by the Circle or Arc tangent command, and not by "Circle through 3 points", but circle and arc tangent are set up to make stuff tangent to 2 curves and through a point, or tangent to 2 curves with a specified radius, or tangent to 3 curves, but not tangent to one curve and through 2 points like you want to do here.
Pilou, I think your method will only work in the special case where the 2 points are equal distances away from the center line...
It looks like the link bemfarmer posted above has the answer in it, though.
 Michael
