I have been waiting for these threads to start. Maybe our friendly Mutoh's contacts will throw a bone to our mutual customers. If not, you could weight your cartridge as full, then print an exact print of x number of inches. Multiply the area by the print resolution (i.e. 1440x1440 times 1" x 1" would equal 2,073,600 drops. Then measure how much weight was lost and divide.) - Note, this would only work for a fixed dot mode. Variable dot mode I would recommend getting the data from Mutoh if possible.
Good luck. As we obtain new droplet size data we will be updating our drivers to reflect accurate default drop size.
Bryan