To quote
javelin:
Sometimes, you do your best and still get low on fuel.
Too true - and lucky's the flyer who hasn't found himself in that situation at least once in his career despite the most careful and thouough pre flight planning.
However, misfortunes enroute leaving you short at destination should have absolutely no effect on the figure you select as your last divert fuel. That figure remains the same - it's simply pyhsics. The engines stil burn the same amount of fuel.
Your bad luck enroute simply means you reach that point where push reaches shove somewhat earlier than you would have otherwise. In a worst case situation, a decision might have to be made to go straight t your alternate without attempting an approach or to drop in to an enroute diversion field for more fuel. Alowing commercial pressures to impinge on that final fuel figure is nothing short of insanity.
A very good rule of thumb that I find works really well for me is to ask myself how I would explain my decision, not to my chief pilot, but to some slick, hostile lawyer who knows nothing about flying but who's carefully read the regulations as he prepared to nail my sorry a**e to the wall a court of law.
Someone earlier said that I overcomplicated the decision process in my long earlier post. Guilty as charged - but I reached that stage of spelling everything out in detail after being repeatedly amazed at how incredibly ill-prepared some of my colleagues are in their fuel planning. How many times have any of you heard an FO say "Oh, we'll need
about six and half tons for 'x'?" - or not include a divert figure in their approach briefings at all?