SmokinLoon,
Where did the OP wish for DT with less than 100% option? and you are wrong. Your gradfather flew planes that were not intended for air combat. Fuel load made little difference for them, so taking extra is a safety measure. Still, some bombers had to trade fuel load and bomb load to keep within weight limitations and in order to carry the maximal bomb load had to take less fuel.
Fighters do care about how much fuel they have in the tanks and in which tanks. However, planes are usually designed for their mission, so a P51 was not used as a short range interceptor. If it had, I am sure they would not load its aux tank for example. The russian planes had a fuel tank the size of a vodka bottle. Had they not filled it to the rim, the planes would have the range to take off do a couple of circuits and land. In aces high we use the planes in a-historical missions. Any combination of loadouts is fair, as long as it was possible in reality without any modification to the aircraft. Could you fill a P51 to 50% and then attach two DTs, without any modification to the aircraft in reality? Of course yes. If it offered any advantage in real life it would have been used this way. P51s never were assigned to such missions where it did offer an advantage.
(The P51 is just used as a typecast)