usually i can estimate the required fuel amount by the combat radius of the mission.
here is what I do:
if the radius is less than 140 nm, no external fuel tanks are needed. (except for CAP, because you need to stay on station for a while)
between 140 and 200, one external tank. But i usually only use centerline tank for the MLU, since you won't have ECM if you carry the centerline tank.
between 200 and 300, two wing tanks.
more than 300, use all three tanks. Again, I usually use two and assign a tanker for the C models in order to use ECM.
Also, sometimes I would purposely not carry any tanks to carry more weapons, and I would refuel either on the way to the target and/or on the way back. you can assign a tanker mission if none is near your flight path. And monitoring your fuel status while in flight is also a good habit to get into. well, those are just my rough estimates, and I'm sure the documents posted previously have much more details, but hope it was helpful anyway.