Not sure why anyone would need that much fuel as in the TA it is 326 minutes at 3k alt. That would be 163 minutes in the main arenas with a 2.0 burn multiplier, or darn near three hours at just above sea level.
For an efficient usage of that much bomb load, you easily get towards those numbers.
Allow me a word on loadout selection first:
guess most occasional B-29 pilots simply chose the 40x500 loadout because it's the larges one in terms of mass and destruction. Only to waste all that awesome power by totally over saturating a target area.
For many sorties, a different and potentially much lighter loadout makes much more sense if you bomb with your head, not your butt. A pilot using the 12x1k loadout for factories AND city, or the 8x2k for factories or the 56x250lb for the City alone and bombing with precision instead of carpeting, will have a much lighter plane with less climb and transit time.
If you take the 40x500lb loadout (or the 56x250 one), you will have to make a lot of passes over the target to make full use of that number of bombs. In this case, 100% may be very well justified. Especially as with the 40x500 loadout you need a lot of time to reach high altitudes (75 minutes to 30.1k from sea level).
The other case coming to my mind is when attacking retreated strats on a huge map like compello. (Such a mission resulted in my longest bomber sortie so far: Compello, 12x1k, attacking retreated strats in the west, total mission time 2 hours 50 minutes.)
Generally, I took 75% for all my strat raids except for the 40x500 loadout (rarely chosen) and the 56x250 loadout, both at 100% fuel. For a strat attack, 50% can suddenly become a mighty tight budget if you got a fuel tank hit and a couple of Ta 152Hs are following you home...