Custom Sales order field with calculation result.

Morning all,  For every item on our inventory (including kit codes) we have a cubic size associated with it.  On the sales order (and probably purchase order as well) I need to have a field that contains the total cube for the order, so everytime I add an item the field recalculates with the total. Like the value of the order field.  I would prefer not to have the cube on the line detail shown on the order.  If a "live" recalculation of the field is either not possible or slows the order entry down to much, then calculating the