When you get the message to replace the carts because the ink has run, out you'll then know how much ink you have used.DOH
Ok ,I'm a wise guy.
Just average it out over a period of time according to the stats in the printer on the actual square footage of paper and ML of ink. Or overall paper (media), to how many carts you've replaced, kinda like figuring your gas milage.
Once you have an average it doesn't really change.
I think it is really nitpicking to try and figure out the xact amount of ink used on each and every print!