Carry what you think you need of course. However, looking at some old flight plans I keep when I land with less than 4.0 (Boeing) and some recent ones......the OFP flight plans you from Desdi to land on 30L to burn only 900 kgs. Nine out of ten times when arriving at the "push" times the Boeing will burn anywhere from 2.0 to 2.5. And this is after the holding or whatever. Next time check the fuel at Desdi and then see what it is on landing. Again, this is just at the high density times. On checking the arrival from Desdi to 12L, the standard flight plan is for 800 kgs. What do you think you're going to land with if the runway changes to 30?
Just a couple of thoughts. So right from the beginning, most flights will use up all of the contingency fuel, not to mention all flights going in the same direction usually on the same routes and getting flight planned altitudes can be a challenge.