Max,
Check your company Ops MAnual adn you'll probably find that the fuel requirement for any sector will be based on a satndard formula which will include trip fuel, contingency, taxi, altn etc. All of the figures are straightforward including the trip fuel. It has to be based on the flight planned route. You may know that you will get several direct to clearances which will shorten it or that you will arrive from the west on to a westerly r/w rather than an easterly etc but there has to be a baseline you can work from hence trip fuel on the log. You can reduce the trip fuel by using the correction block on the flight plan if you wish but bear in mind you don't have a section on the fuel plan for the standard TMA delay of 20 mins or so.
As for being over the ldg weight because you havetoo much fuel on board, you can always work out the max fuel to load to avoid that by using MLW - MZFW + Burn Off = Max fuel load.