I find it easiest to use either the stats page or DaveB's FSNavigator fuel settings to work out how much I need - plus the usual 'bit to be on the safe side'
DaveB's settings are in a sticky post here
The refuelling part of the Flynet client deals in kg by default (I think) so even though I'm a lbs and ozs man with Flynet I go metric

Rgds,
Colin