When on final glide, call someone on the ground as you start dumping, then have them note when they see you've stopped. Do this at a few different speeds to see if it makes a difference. Then time a dump on the ground for comparison (be sure to set up the plane in flying attitude as when doing CG).. Finally, put the plane on a scale and note the weight vs. time.

Or, just make up some rough numbers and I'll bet the computer will give the same results either way. :-)

Oh, and remember to weight yourself just before the flight, and keep track of how much you pee if dumping it overboard during flight.
;-)

5Z