If the base surface is flat, it doesn't really matter if you draw the polygon outside it a bit imprecisely. With the base defined along the flat surface no volume will be added to the total for that area so it is OK to draw outside the lines a little bit on that.
We usually draw the stockpile perimeters with the elevation map showing so we can clearly see where the stockpile begins and the bare base ends.
We already allow the map to be overzoomed one step beyond native resolution. It looks like a lot of your jobs have a reduced resolution for the source images which is making the GSD really high. Not only will the affect your ability to zoom in but it will affect the accuracy of the pile's volume calculations as well.