Try this:

         PointF p1 = new PointF(262, 47);
         PointF p2 = new PointF(262, 254);
         List<float> distances = new List<float>();
         for (float x = 250; x < 270; x++)
            distances.Add(Utilities.DistToLineSegment(new PointF(x, 121), p1, p2));
 

As x approaches the line the distance gets smaller, and when x has passed the line the distance gets larger. But when x is right on the line the distance is 74 instead of 0!

I think I've spent something like three hours trying to find out why the mouse selection of a custom-drawn node was unreliable. The problem is if I click exactly in the middle of the node then it didn't work, but it took quite a while before I figured that out, and then a long time tracking down why it was like that.