That's pretty cool. So it's showing 4 gal? what do you have it in?
We always just timed how long it took to fill a 5 gal bucket to 5 gal. measured with 5, 1 gallon fills. Then used that to determine how much water we were adding by time. That would be handy because you don't always fill from the same place and you aren't always setting on level ground.