The issue here isnt the 30 minutes. The issue is that if you are going to burn any of the fixed reserve you have to divert or declare an emergency. Ie there is an ongoing obligation in flight.
The USA rules make it a requirement to have 30 minutes of reserve fuel at the beginning of the flight. After that if you need to use it you can.
From 14 CFR 91.151
(a) No person may begin a flight in an airplane under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed -
(1) During the day, to fly after that for at least 30 minutes; or
(2) At night, to fly after that for at least 45 minutes.