So you are looking for it to change the color of the lowest point in the series of points in the polygon? I think it is definitely worth looking into, but if I understand it correctly I don’t quite see how one point is going to define the entire toe or how it would be able to change dynamically as you pull it across the surface.
There are several methods that make it quite easy to locate the actual toe of the stockpile. Distance profile, Elevation Layer and 3D Takeoff.
Distance Profile - Create a perpendicular annotation and see where the toe falls in comparison to the mid-point. This was an older method that we used if we were unsure before the following two methods were available.
Elevation Layer - This is probably the quickest way to see the stockpile in a manner that makes it obvious where the real change in grade happens. The trick with this one is that when you are on terrain that is very sloped or the change of grade across the site is severe that you can adjust the sliders to get a more relevant gradient on the area your are taking off.
As you can see this site has allot of relief so the stockpiles are not very defined when the elevation profile is spread across the entire site.
You can then adjust the slider bar to get more definition.
3D Viewer - This is best for confusing stockpile shapes or if there is grass or silt fence around the stockpile that makes is more difficult to hit a low area. Just drag the vertices around and you will see it change elevation. This is particularly good when stockpiles abut each other and you have to find the low point or jump across. We typically set by the elevation view and then tweak with the 3D view using the cut fill gradient to remove any fill areas.
As you can see this stockpile is quite irregular, but it is also to tell that there is very little blue area so your reported quantity will be a more accurate cut value. You can also more easily see the toe of the abutting stockpiles.