IO540: I use commercial reserves - even when flying a PA28. I.e, 5% contingency fuel, plus enough for a go-around, 30 minutes "holding" fuel, and a diversion to a suitably planned alternate. A bit like an airliner's Computed Minimum Reserve if you will. It usually ends up at around 7.5-8 US Gallons (depending if it's a Cadet, a Warrior or an Archer) leaving about 4 hours' endurance (even without leaning). That still allows to fly beyond Le Touquet.
This still requires accurate fuel flow information. All the planning in the world will not help if this is not known. Any modern airliner has accurate flowmeters (of the order of 1% accuracy) and I believe most modern ones also have accurate fuel level gauges.
Incidentally, the fuel flow difference between a) full-rich and b) leaned to peak EGT, is about
30%..... a LOT. Most UK PPL training is done full-rich, which robs the plane of a large chunk of its useful range for going places.