With fuel planners supplied with the plane such as Quality Wings, Maddog and so on, these contain lookup tables based on the aircraft flown in the sim - THEIR sim!
With Ideal Flight FIRST we fly the plane as a test and IF measures the fuel used, the measurements and the de-brief come with a fuel Range calculation based on the conditions within YOUR sim the way YOU fly!
Following that you create a flight based on that range calculation - we enter the range climb descent cruise values and so-on in IF. I make flights with a full load and a half load - and stick to the same climb, descent, cruise and weather conditions and see the difference in fuel used.
You use this to properly understand the fuel the aircraft uses, the way YOU use it. That is if you are interested in fuel calculations. The full suite of BOEING recommended calculations can be found near the end of the briefing.
This way is the most accurate way to predict fuel bar none. Only in Ideal Flight.