The most important part of the fuel calculation is that it's stored to DB in lbs, so all you need is to fetch the fuel amount from DB and show it where ever you like. Also, what i can offer is a module I created to calculate fuel, based on aircraft parameters (ex. fuel burn per hour, altitude,...). Then it will give you an estimate of how much fuel to be loaded for a flight in both metric and imperial systems.
Here is the link: FuelCalculator_V1.0