Do you have one of these?
With modern fuel flow instruments it is easy to be brutally accurate in your fuel quantity on board. Several aircraft I fly have these systems and landing with just over 30 minutes fuel (in other words less than 1/10th of a tank remaining) is a regular occurrence. Failing that, you can be fairly accurate starting from full tanks and using a stopwatch for fuel burn but it takes some practice. Otherwise you just overestimate for W&B and underestimate for FOB and hope you aren't too far off.