How much water you can get in is dependent on how level the tank is and what direction, the position of the overfill port on the tank, the routing of the overfill drain hose or pipe, the temperature of the tank (it's plastic!), etc.
And those P3 meters have a stated accuracy of +/- 1/10 th of a gallon! This is 10% so the meter could read 50 gallons and you have 45 gallons or 55 gallons.
The manufacturer's of the plastic tanks have a +/- also on what the tank can hold, notice that most say NOMINAL which means it will vary.
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