Hi Marc  so normally when placing the cplane if you pick a point on a general curve (planar or not), it will align the z axis of the plane to the curve's tangent direction.
That's so you can do things like stations along a central rail curve, something like this:
But if you snap on to a center point object snap on a circle for example, it will align the z axis to the circle's plane normal.
So possibly the easiest way to get what you want is to draw in a circle with "circle from 3 points", with each of the circle's points snapped on to that curve, that will put the circle in the same plane as the object and then you can place the cplane on to the circle's center point. The cplane's z axis will be aligned how you need it then from the start, and you can drag the x axis to point it towards the direction you want to have as the horizontal, it will pivot around the z axis.
You may find it easiest to place the 3 circle points in some localized area like this (possibly turn off straight snap):
and then move it around after that.
It would be nice to have an option to place the cplane in the same plane as a selected object, as a different way to place it than by picking a point, that's something that I could add in v3. But for the moment drawing in a circle should do the trick, let me know if it does not.
 Michael
