Re PA28 fuel gauges - almost useless.
Re dipping the tanks...
Well, you could try the method above, but beware of the following:
Try going through the above exercise, then with fuel well below the tabs, move the aircraft to another location and try again. I bet the results will be different. Getting the 2 wings level is a nightmare. Firstly, hardly any surface is level enough and you can bet your bottom dollar that one surface will have a different slope from the other. You can't guarantee you'l be making the measurements on the same slope every time. Secondly, you'll not get the oleos to settle in the same place.
Try this. Get a tape measure. Carefully measure the height of each wingtip from the ground. Get hold of one wingtip and vigorously heft it up & down a couple of times (like you do during your pre-flight check). Re-measure. I bet you get a different result. I also reckon your dip will give a different result, too.
To my mind, the only sure method is to measure usage over a period of time, then apply this to the time flown since the last tank-fill, then apply a comfort factor (at least 10%).
Our Warrior 1 does about 6 hours from full tanks at about 2250 RPM, suitably leaned, about 4 hours from the tabs. Personally, I never go below 2 hours reserve, just in case, that's 4 hours flying from full, plenty I'd have thought between loo breaks!
My favourite application of a sawn-off broomstick is in the story about Chuck Yeager. About to go off to break the sound barrier for the first time, he finds he can't lock the door 'cos he's cracked a rib falling off a horse. So his trusty engineer saws off a bit of broom stick just the right length to enable Chuck to close the latch. It's all in Tom Woolfe's book 'The Right Stuff', so it must be true. (Brilliant film, too)
Cheers,
The Odd One