Find out average fuel burn of aircraft type. Find out average cruise speed of aircraft type.
Divide fuel burn by cruise speed to give average pounds/kilos of fuel per mile .multiply the route distance by this number to get zero wind average route fuel burn .
To the answer above add half the average hourly fuel burn as a final reserve .
Multiply distance to ALTN by fuel per mile and add to answer above .
Use the trip fuel burn to calculate what contingency fuel you need .add that to the answer above to determine min zero wind fuel for the route. Adjust for wind if needed.