Can't possibly be vary accurate for one reason, the length of the gas bubbles is not a constant, it will vary as temperatures and air pressure vary. it might come close but at best t's just a very rough estimate.
If you want to test the flow on any given day, time how long it takes a container of known size it takes to fill, but realize that will only give you the flow right then, it will usually vary every day, even at different times during the same day.
Just look at the flow, but be careful, it might hypnotize you.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.