David
I am using the OpenRoads tools in PowerCivil 8.11.9.845 - which are the same as you have in GEOPAK (and MX, which I normally use).
Anyway, the attached PDF is how I calculated the cut volume (at least, what I think is the cut volume you are talking about!) using two different methods.
Hope it helps.
(Please visit the site to view this file)