ijpdesign

I am currently modeling speaker cabinet designs and internal volume is critcal in the factoring of drivers etc. I am wondering when vectorworks calculates volume for an object if if uses the outside planes of the object or if when using extrusions the volume is calculated from the interior surfaces that have adjoining surfaces? Or will vectorworks even calculate an internal volume such as this. The file is a very over simplified version of what I am trying to do. The green represents the internal volume and the red is the external.spkvolume.vwx

I am trying to bend a piece of steel 3/8" x 1 1/2" for legs of a table. I draw the rectangle and extrude along the leg path, my problem is where the extrusion ends there is a taper on the leg. I have tried keeping the profile and locking with no luck. Any suggestions would be a great help