This is an interesting thread for me, someone who did his PPL in France.
You see, us cheese loving monkeys love to stick to the traffic pattern in VFR. Almost every airfields have an official or unofficial arrival/departure points that join the traffic pattern and pretty much all VFR AC have to go to these points before they're allowed to start their nav.
In terms of fuel calculations we always approximated 30min of cruise fuel for T/O, climb to what we called "departure altitude" and reaching one such point. The same for arriving at an airfield, 30min. Only exception would be when there was a considerable distance between these points and integrating into the traffic pattern, in which case a more precise fuel calculation was done.
Never ever ever heard of calculating navs by starting overhead the departure airfield.