Best Method for Propane Level guesstimates?

Here is my noob question of the day: what is the best method for determining a guesstimate on propane tank level?

I have that sensor and the only problem with it is when the gas level gets in the range of the tongue bracket band. In that case, you just have to check the levels above and below; if above is red and below is green, you know the gas level is somewhere within the width of the bracket band.

The warm water side is probably the cheapest and most accurate method. Don't spend extra on tanks with gauges. Green, yellow, and red are only relative with these gauges. Red does not mean "empty" but merely "low" instead. This makes the gauges pretty much useless as on our trip out west, we had another day or two of use out of a tank where the needle had bottomed out in the red.

The ultrasonic reader is a nice high-tech gadget, but I would not say it is going to be any more reliable than the warm water method.

But, that's not convenient for a tank strapped on the r-pod.  I've got a couple of propane tanks with built-in gauges.  They didn't cost that much more than regular tanks.  They gauges aren't perfect - they show empty when there's still 2-3 pounds left in the tank; but, they're not that bad.

Built-in gauges are somewhat useful, if only to give a relative reading of tank contents. The warm water method is good because I don't have to take the tank out of the holder to use it.