Backpacker's advice is the one you want then. NEVER believe what it says in such flight manuals as these give the best figures with a super clean aircraft with immaculate paintwork, a just run in engine and a test pilot at the controls. In real flight manuals, prepared for public transport aircraft, such performance figures are "factored" which means that they are made worse to reflect an aircraft in normal condition flown by an ordinary pilot.
Rememember the old adage:
There is always plenty of gravity:
How much fuel have you got?
Professional pilots use factored data to calculate the fuel that, including reserves, they need for a trip - then they always add on a bit extra!
P.P.