Hi Aumonier,
In 99.99%% of cases, the fuel the company (or more specifically, the Flight Operations department) gives us sufficient to perform the flight safely and arrive with above legal minimums (30 mins holding fuel). Sure we can use an average fuel flow to perform a quick gross check but these days more commonly we need to take other matters into consideration. In order to produce a BLOCK fuel figure, the route the company thinks we are going to fly needs to be accurate. It used to be that they all used to plan fuel for the 'longest' departure procedure and the 'longest' arrival procedure. Nowadays, they don't. They use what they think we will get based on weather or past patterns they have observed. If they insert a shorter route into the flight planning system (Jeppesen or LIDO are the two most popular ones), then they will get a smaller amount of BLOCK fuel. We therefore need to sometimes uplift more fuel to match what we think we will need on the day. Secondly, we need to observe the weather and bear in mind traffic at the destination. These may require us to hold, more fuel needs to be taken for this.