There is no need to use the Ground Offset unless your piles are super huge. As long as you are flying 5-6X higher than the highest point and are using 80% overlap it should be fine. We always recommend using at least 80% overlap when you care about the accuracy of the 3D models. For measuring stockpile volumes this is all you care about.
The void is not the void inside the pile. The void is a measurement of the air open air above the surface inside the measurement polygon between the surface and the peak height. This value can be used the measure the volume of a pit.
We are working on a new reporting system.