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Yes, if the wing/tank arrangements are the same between your P model and my normally aspirated model, then if you see a thin film of fuel under the filler hole you have 30 gallons, and the tube (after you calibrate it) is used to determine quantity when you have more than just a film (1/4", 3/4", 1.5", etc.). The airplane must be on a level surface for accuracy, and, as mentioned by others, you need to wait until fuel has settled because the fuel SLOWLY feeds down to the inboard part of the tank system.
Filling is important, also. If you're starting from a very low fuel situation, use of the visual-and-tube technique on the first tank/side you fill will give you an inaccurately high reading, because the weight of the fuel on that side (unbalanced by the emptiness of the other side) will make the aircraft tilt in the direction of the tank you are filling, bringing more fuel under the fill port than there would be if the aircraft was level. So, when you just barely see fuel under the port (after waiting for fuel to settle, of course) you won't have 30 gallons, but 28 or 26 or perhaps less. Same with the tube. After calibration, if it shows you have 50 gal, you might have 45. The solution is to go fill the tank on the other wing and then return to the first tank to re-measure and refill as necessary. Generally, this iteration is not needed on the second tank/side, just the first.
Ernie
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