Just a couple of points following up all the interesting points raised.
Aztecs join the group of aircraft whose tanks appear to be dry when plenty of fuel remains, but how much exactly lurks in the tank is unknowable because you don't know how much more fuel was used after the initial 'dry' point, and who trusts aircraft gages to tell you.
Fuel flow gages are fitted to injection engined aircraft, but like all other gages they are not reliable within a couple of gallons per hour, so not suitable for fuel planning.
They can be cross checked by keeping a fuel log, and compared in a twin by referring to the EGT, however any gage can play up during the course of a flight, and leakage and theft can upset the log.
Reserve fuel tanks. We all have reserve tanks, they are that portion of the fuel load that should be reserved for in fuel planning, and that sort of reserve doesn't go stale.
Head winds, strong enough to throw marginal fuel planning out the window, can be overlooked, I was highly surprised at 90 kts ground speed in the Rhone valley one time, and that was with 160 kts airspeed.