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I used to use a clear plastic tube that I marked in five gallon increments. I emptied the tanks (to the point where the tube did not touch fuel), then added fuel slowly, marked the tube, then made a backup tube.
On my '73 P337G, a good amount of fuel was in the wing roots, below the level of the bottom of the tank at the outboard point where the filler neck is. So if the tank looked "empty" out there, the airplane had between zero and (as I recall) 35 gallons of fuel per side. 70 gallons is a bit over 2.25 hours of flying at the power settings I used, so I never found this to be an obstacle. If the fuel level was below this point, and I wanted to stay light, I would just add enough fuel to make it visible, and then I would know I had a bit more than 2 hours of flying time available. (Hope that paragraph makes sense.)
In my '65, determining fuel quantity was easier, since I could fill just the main tanks (and not the aux) and have a defined amount of fuel. I had a dip tube for that airplane too, but I did not use it unless I was really putting a lot of load in the cabin.
Kevin
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