I recently got to measure the flow rate using the inverted measuring flask method and then compared that rate to my own tanks as well as a client's.
The predicted value for my tanks rate per gallon was exactly the same as the rate I set for for the client's.
Furthermore, the Mass flow controller's rate was also exact, in this case 14 mls per second(roughly 1.000+ on the flow reading measure). Very large tank though.
This can be correlated to a pH/KH/CO2 chart since the tank water is 100% RO with baking soda.

What I am not certain of is if the relationship is linear. Eg: would the Mass flow controller readings of 0.250 equate to a 0.25 * the original CO2 ppm?
It would seem like it. I have not yet had a chance to verify it.

The end goal here is to know the size of the tank, and be able to use other reference tanks to determine a good flow rate of CO2 (something anyone can measure with a flask or measuring cup inverted in the aquarium water).
If they use a wet/dry and over flow type of filtration and what not, the degassing rates should be roughly similar/decent flow etc. So the tanks should be fairly close.
Now the 4th thing would be to use dry ice in a sealed 4 KH reference solution to make a reference 50 ppm CO2 solution standard.
Then I could verify each method. Ideally, they would all match if the user's methods are done well.