Because of the possibility of a tank leak and fuel theft overnight, I never take off without a visual check. While I have and use the plastic/glass calibrated tube, after a while you hardly need it. On my 337G with the extended tanks, if I see just a bit of fuel in a tank, I've got 30 gallons. Full it's 64 gallons. And you get pretty good at estimating in between. Plus, repeating what I said above, I never take off unless I see and can measure (or estimate) fuel. That means that my flight planning includes putting enough fuel so that when I get to an airport without fuel (the norm in the Bahamas) there is at least 30 gallons per tank (so I can see fuel).
One other thing. I don't think the issue of non-horizontal surfaces renders the tube ineffective. In virtually all airports or air strips, I think you can find at least one 12-foot diameter area that's horizontal.
Ernie
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