There will be company “minimums” as well as regulatory final landing fuel. It is not often the two are different these days. In addition, the contingency fuel is at the discretion of the commander to load what he feels based on..... and then “some for granny.”
As for estimates, that’s exactly what they are, based on planned winds and altitudes with both forecast and historical wind data to give a rough (but good) estimate of trip burn. However, winds are never as forecast and often ATC will give a different altitude to that planned. All of which cause burn and time differences. Sometimes we have to plead with ATC for higher levels as the assigned level against plan starts to look “not so clever.”
Usually, on our trips, planned burn is often within a couple of hundred lbs of plan. But we monitor it regularly and when the calculated landing fuel starts to trend away from plan we’ll re think as appropriate.