I round up the POH fuel consumption figures, then I round up the planned time for the flight, I plan to land with a full hour's fuel even for a single VFR circuit and even over East Anglia when there's always a choice of a dozen runways within twenty minutes, then if the aircraft has one gallon less than what I've calculated I need I add fuel.
One the one hand I do occasionally get told I'm being over cautious (I accept that people who are better at planning and flying than me won't feel they need the same margins), but on the other hand I've never (yet) run out of fuel.
And people do run out of fuel, as we all know, and sometimes they die.