Gradebook

Hi all. We're upgrading our current 1.6 Moodle to 1.9, and of course this means using the new gradebook. Currently we are using Calculated Columns, but obviously with such a big change in the gradebook we can't really upgrade this directly. So my questions are:

• is it possible, using the standard gradebook, to add a 'hand in rate' column? Currently this is quite a simple formula (round(step(all)*100/N)), but I can't see how to do this with the new gradebook.
• if it isn't, I want to start updating Calculated Columns to work with the new gradebook -- but where do I start? I'm no stranger to hacking Moodle, but this 'tree' of 'grade_objects' has got me flumoxed. Is there a guide to the internal structure of this aspect of Moodle somewhere?

Thanks a lot, Moodle gurus!

Tom