I can imagine that would be a very inexact science.
Not at all, and couldnt be simpler.
After fueling (to the brim) you note the gallons paid for.
Then look at the Flowscan measured gallons used .
If they disagree a tiny dial will reset the gallons used to closely match the fill amount.
You then reset , and you are done.
For greater accuracy , dont reset and simply allow the meter to keep counting as you log each fuel,fill.
Eventually you will fill to the brim again and adjusting for 1200gal will give more accuracy than a couple of hundred. After a while its a big game , till it ends , when the fill and measured match.
Well under 1% accuracy can be got with patience.
We fill say 160 gal in two tanks and the gauge shows 158 burned , no big deal to me.