Owen it really depends how deep into the planning process you really want to go. Simbrief is pretty much a freeware equivalent to
PFPX - Professional Flight Planner X.
Here you can calculate EDTO range circles and fuel as well as alternate reserves and contingency fuel. It can use real world winds and nav data. You can also apply a sort of "performance deterioration allowance" correction in the drop down selections.
In RW aircraft this is used to apply a correction to flight planning book figures due to inservice deterioration such as high hour engines or aircraft that develop more drag as they get older (warping, dirt and grime build up, airframe patches and additional aerial installations etc). Its very similar to a car using more fuel per KM as it gets older. So with Simbrief you can tweak this figure according to your flight experiences to get some very accurate fuel planning figures.
If you just want a basic idea of how much fuel you need to get from A to B then you can just fuel the aircraft until the Fuel at destination, on the progress page, indicates a sufficient amount for a reserve at destination and off you go.
The beauty of being able to print out a proper flight plan (and it is available in many real world airline formats) is that crossing an enroute waypoint you can compare the fuel on board with the minimum fuel required at that waypoint. This will give you a surplus figure (hopefully) and you
can compare this to other waypoints and see a trend forming ie. losing or gaining fuel. Very good for time keeping as well for things like VATSIM online flying.