I used to start from OH for PPL students but then decided it was not a practical way of doing things for once people have got their licence so why teach that in the first place? By the time they get to Nav most students will be used to flying in the local area and back so I now teach flying to a landmark en route and just outside the ATZ and starting from there. For fuel In most light training aircraft I normally suggest allowing a gallon to this point, this will normally be enough to get to a point 3 miles away including enough for the climb and means you can then calculate the rest of the flight at a cruise fuel flow.