Another problem with assuming stoich is that even 'normal' cars don't run stoich when cold. My hunch is that this is another factor that introduces errors, for all cars, into the data SG displays.
Another problem with calculating consumption indirectly (instead of looking directly at the injectors) has to do with DFCO (deceleration fuel cutoff). I think SG only knows about this indirectly, because the user enters a constant representing the known DFCO threshold in rpm. This probably works well in most cases, but there are things that can go wrong, so it's not as good as watching the injectors directly.
A good discussion about mixture issues on diesels is here:
http://www.eng-tips.com/viewthread.c...=130502&page=1