I don't know, I just measure it with the gas pump
There's lots of variables as I said. I would just top off the tank and fudge usPerGallon so that mpg looks about right, then true it up at the next fillup.
But in essence, it is the pressure drop across the injector that determines the flow. If you know the operating pressure drop and adjust the published flow rate for the operating pressure drop in your vehicle, you can multiply that by the number of injectors to try and predict how much flow per time total.
Also, you did not mention what vehicle you have. There is a known issue with peak and hold. Just as a precaution you might want to unplug an injector and measure it's ohmage, if it is above 10 ohms you should not have a problem.