All methods that I am aware of that check total ink use take into account the blank canvas/artboard area, so I am guessing that the formula requires a bounding area of some sort vs. the object area. The larger the bounding area the less ink reported, which may be misleading and why I mentioned this in my OP as some may not be aware of this.
PitStop’s ink reports are smart enough to only report on the ink/area of the “visible” masked shape, so a 100x100mm mask of a 200x100mm spot colour object will only report as 1000mm2 (100 x 100mm as the final masked size, not the actual object which is 200x100mm). I am presuming that overlapping panels may skew things a bit, one would need to use a pathfinder type function to join them, however if the overlap was small then this would not be too big a deal.
Stephen Marsh