If you select 25% in the hanger, you'll get 3/8 internal fuel, with empty wing tanks.
If you select 50% in the hanger, you'll get 3/4 internal fuel, with empty wing tanks.
If you select 75% in the hanger, you'll get full internal fuel, with 1/4 full wing tanks.
If you select 100% in the hanger, you'll get full internal fuel, with full wing tanks.
I like to take the full fuel option for "normal" flights. I just programmed a button to rotate fuel tanks, so I can burn my wing tanks first. I also mapped my keyboard so just plain "F" rotates the fuel tanks. I alternate between the L/R tanks, burning them off fairly evenly (about 1/4 tank at a time) until I reach about 1/8 in the right, and empty or 1/16 in the left. Then I switch to the main tank for the rest of the flight. That leaves me with barely enough to RTB on my wing tanks if someone wrecks my main. I also like to have the right wing a little heavier than the left, so I leave some fuel in the right tank. I don't like to burn the left tank empty before burning any out of the right, because I don't like the right wing to be so much heavier than the left. If I get attacked I want fairly evenly balanced wings.
If I fly with 75% fuel, I do the same thing. I fly that way if our base is being attacked, but the enemies aren't vulching yet.
If I lift from a vulched field, I'll take 50% and a DT. I'll get rid of the DT as soon as I get attacked (maybe even on the runway...), but will try to "sneak" out with it and use it to climb.