What's the big deal ?
It is really very simple. Contingency covers unexpected situations. For all the rest you need to carry the fuel. If you go to LHR at a certain time and day of wich you know it is reasonable to expect a delay then you have to carry the fuel. You can not use your contingency for that.
The seabreeze in DXB is very predictable .... so 30 L is what you take.
LVP simply take 30 min
NP simply take 30 min if vis is lower than MDH x 6 .... that is what you need to see the RWY from your MDH
OFP gives you a constant decent while reality is the opposit .... take some extra.
After every flight keep a copy of OFP and WX and reflect on your decision. Soon you will get very accurate. Make it a habit to bet with yourself what your landing fuel will be at the moment you decide your uplift.