As far as I tested the accounting seems to be accurate enough for prints, but it ignores ink wasted during cleaning, head swap and calibrations. That is why I weight the tanks periodically.
Actually, I know that this is completely wrong. I had major problems with my new 6400 and needed a new CPU board and carriage. I was already monitoring my ink levels and its amazing how much ink was used. Here is a screen capture from the software. These were only two days apart, and you can see that there was really only one canvas print made, maybe it was just testing, I don't remember, but look at the huge difference in total ink. This was after a first head change, which didn't end up fixing the problem and hence I needed to wait for new parts and stuff, but you can see the difference in ink is about 206ml. If I assume 10ml of ink for the prints, then its just under 200ml used for the head change. If we assume 6 color channels per head, that works out to about 30ml of ink gone for each color to do a head change, which sounds about right.
Just the other day I got stuck with not printing for a few days, so the printer first did a clean cycle, and I could see that about 15-20ml of ink was wasted for the clean cycle. I try to print something every 2 days so that a clean cycle isn't triggered for this very reason. 15-20ml of ink wasted on a clean cycle will make lots of prints. Who knows, maybe the heads needed to be cleaned anyway and it might not use this much each time it sits for a few days, but the point is that this menu setting shows all ink used.
EDIT: In fact, as I analyze this, you can see that the colors that hardly change, 48ml to 50ml, were on the side that needed have the head change, which are the bottom 6. Now the top six mind you still don't really change in a uniform amount, so I'm not sure what to make of that... hmmmm