... what I do know is an almost guaranteed full gauge that works on most trucks and buses is a fuel stick or hose! (if the neck bends and a stiff stick won't make the curve a semi-flimsy hose with a weight of some kind on the end works well to drop in the tank until it bottoms out and then pull it out an see how much fuel you have in inches. Then compare that to the depth of the tank in inches.
Good points, there, BK. I'd add that most tanks have the pickup a "little" off the bottom so it doesn't pick up trash and water off the very bottom of the tank. Also, if there's a pinhole or broken piece at the bottom of the pickup, it will leave more fuel in the bottom of the tank when you use all your "useable fuel". To be really sure, you should pump out your tank into a can (prolly real good to start when you're near empty anyway) then measure from what's your "minimum pickup" level -- but I don't have the patience for that and I can't imagine most people doing it. But just saying ...
I like your flimsy-hose idea but I have found that a cut-off remainder of Romex is good. It will take markings with a permanent pen pretty well, or you can scratch graduations in it as you measure. And it's flexible but it also "take a set" if you need to bend a curve in it. Of course, like most other things - nothing works on ALL buses.
HTH, BH NC