I struggle to get the volume right for the grading group in the attached drawing.
After I create the surface from the grading group then the tooltip reports Z=0 for the surface created.
Also the volume is reported huge and I cannot get to work the auto balance from the grading volume tools.
I am having trouble opening your drawing. It is crashing so that probably tells me something is wrong. When I do a recover of your drawing the grading group surface does not come in, only the existing ground. You might want to repost the drawing.