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The pump shut-off is designed to prevent the tank from being completely filled...mainly to prevent spilled fuel which is a fire hazard and pollutant.
The fuel nozzle usually has a bunch of tiny holes drilled near the tip. These holes are used for sucking up old fuel vapor. When the fuel level rises in the tank to cover these holes, the pump automatically turns off. There shouldn't be too much of a variance between pumps. If there is, it is probably because the pumps are calibrated slightly differently.
Your method is pretty much the best you can do. You can test the accuracy of your odometer by using highway mile markers, gps, or Google Earth. You can test pump accuracy by filling up a graduated container. Pumps will always vary slightly because of temperature variations (they measure volume, not mass)...but underground storage tanks keep fuel pretty stable.
- LostCause
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