You need a Robot or similar float control mechanism on the combine, with the cable going down to the header, and the linkage with pads under the header. In the field you punch the header lift control (left lever) down to the bottom so it stays there, and the robot controls height across the field. Works well if adjusted properly and the field is not muddy. Helper springs on the header lift cyl need to be adjusted to help control the degree of float. I spray painted the lift cyl when the springs were adjusted right- easy to reset when needed.
Lacking that, I mounted a steel rod on the thresher housing, ran it through a steel loop in front of the bottom front of the cab, and painted the top couple inches a bright color. On the cab window I put two short pieces of electrical tape, one indicating when the FCB was bottomed out, the other indicating when the FCB was just above the ground. By manually running the header I could keep the bar at ground level, even in dusty conditions when I couldn"t see the cutterbar.