Hi Charles,
your design works well if your flying circle is dead flat side to side, but if one side of a circle drops 1 or 2 feet like I have seen on various circles you would end up following the terrain but not flying level at all points of the circle. The answer is a marker that is height adjustable. have a pole in the center of the circle and tie on a string with a line level to set the markers height or use some kind of laser level on the center pole to do the same thing. What you don't want to do is practice following un-level terrain instead of practicing flying level. Best wishes,
Pat Robinson