This procedure is a general method of reporting cut/fill values.
There are other ways to report values, including sketching a new surface within an existing surface.
For example, when the view's phase is New Construction, set the Phase Created value to Existing.
Modify Site panel
In the Edit Graded Region dialog, select Create a new toposurface exactly like the existing one, then select the toposurface. This step demolishes the original surface and creates a copy that occupies the same boundary as the original.
You may need to press Tab to select the copied surface.
In the Properties palette, Revit lists the new cut/fill properties for the toposurface.