+10% (can recalculate the 10% over long flights).
10% may be an unattainable amount for some aircraft; at the least it could prove costly. On the Airbus (and some Boeings), imagine that 10% could initially be over 10 metric tonnes of extra fuel, depending on the flight time. The cost of carrying that fuel, due to increased fuel burn, can be considerable. Also, any time you take-off with additional fuel, you have to consider that you may have to turn around and land with it, or take extra time to Dump it. Not good when things are burning, etc.
Also, that 10% extra could amount to between 20% to 100% (one hundred percent) extra fuel by the time you are within an hour of your destination. That's a lot of extra fuel.
Similarily, on a shorter and heavier yield sector, that 10% could restrict available load resulting from the Maximum Landing Weight.
A more realistic number would be 3% (max 5%), provided of course that you have a suitable destination alternate, and suitable enroute alternates in the event that things go amiss during flight. We regularly use 3% and simply "Re-dispatch" in flight as we pass various points ensuring that we have the required (and safe) fuel to reach the enroute alternates, until in range of the destination, where at the final check point, you'd require 5% of the required fuel to continue to the destination.. Provided that you have suitable alternates, the same formula can be used on any type of aircraft or operation.
Basically, at each check point, you need to ensure that you have fuel to the destination, the destination alternate, and the required reserve (15/30/45/60) mins), PLUS the 3% extra. At the final check point, the Extra needs to be equal to 5% of the fuel needed to continue to the destination. Any time that the numbers don't add up, you divert to the enroute alternate. It's all in the planning and preparation.
And, of course, this is a simplified explanation.