I agree fuel gauges are crap and cannot be trusted - except for very few like the 28V TB20GT ones which are super accurate.
IO540 - that's a very professional and involved way of doing it! Do 2 flights, identical profile etc... all very good when you are an FI, or own your own a/c - much less practical when you rent a club aircraft once/twice a month! I'm not too sure most people would want to repeat flights just to assess the fuel burn. I know I wouldn't.
This may sound a useless comment but you pays your money and you takes your choice.
If you want to fly on the cheap, you should not expect the same mission capability.
Put it another way, if you cannot afford to do a couple of flights, total time say 3hrs, and you cannot afford to do them even with 3 people cost sharing the flights and going somewhere for four burgers, then your only option is to fill right up every time and treat the fuel endurance as 3 hours when in reality it might be 5 hours.
The advantage of knowing the fuel endurance is say 5hrs is that you can now do longer flights, and plan your alternates intelligently. Whereas PPL graduates have usually got no idea what the real endurance is - not least because they have never been taught to lean the engine - so they stick to 1hr-2hr trips. This is OK but it cuts out a huge chunk of utility/fun value out of GA, which in turn results in so many people chucking it in very early.
The difference between 3hrs and 5hrs is that you can do a long trip into France; somewhere beyond Le Touquet
