Our flight plans have columns for fuel remaining and fuel used, we compare the planned vs actual figures. It is useful to check both for two reasons.
1. Double checking.
2. If one figure changes and the other doesn't, it is easier to diagnose the problem.
Example 1 (leaking from tanks / lines):
Hour | Extra fuel on board | Extra fuel burnt
_1__________4_______________1.5
_2__________4_______________1.5
_3_________3.5______________1.5
Example 2 (inefficient engine):
Hour | Extra fuel on board | Extra fuel burnt
_1__________4_______________1.5
_2__________4_______________1.5
_3_________3.5_______________2