The structure of fuel reserves is usually of the order to fly to destination, go around and divert to alternate, plus Contingency (5% or 15 minutes holding), and land with 30 minutes left. As a planning minimum this is enough. As fuel becomes more expensive, you must tailor the cost of carrying any extra to your perceived risks on the day, as that extra fuel you carry every flight will hit the profit. You cannot justify this for the once in a blue moon event of a diversion through low fuel which may cost less than carrying extra fuel on average every flight. Remember, extra fuel mostly doesn't prevent that diversion. On most occasions when I have diverted, extra fuel merely delayed the inevitable.