[BobS]

As UltArc can attest, using a gas pump to get the accuracy a properly run event like the GGP requires is not such a good idea. A simple accurate measure can be made with a float switch, an LED, a AAA battery and a 3/16" aluminum rod. As the name implies, the float switch has a tiny float which rises when it hits liquid, allowing the LED to light.
Fill the tank until the pump clicks off then run this device a fixed distance down the fill pipe and add gas from a gallon gas can until the LED lights. This would still give a reasonable fill rate at the gas station but would fulfill the need for accuracy required by an event of this type. Except, of course, for Crazy Jerry. Does anyone have a calibrated eye dropper in their medicine cabinet they could donate?