var pt = (duct.Location as LocationCurve).Curve.Evaluate(0.5, true);
var plane = Plane.CreateByNormalAndOrigin(doc.ActiveView.ViewDirection, doc.ActiveView.Origin);
var planeUP = Plane.CreateByNormalAndOrigin(doc.ActiveView.ViewDirection, LeftDownModelRect);
SketchPlane skpUP = SketchPlane.Create(doc, planeUP);
PointOnPlane pointOnPlane = PointOnPlane.NewPointOnPlane(doc, skpUP.GetPlaneReference(), pt, plane.XVec);
System.Windows.MessageBox.Show( pointOnPlane.Position.U + " " + pointOnPlane.Position.V);