Rolfe, I think you are confusing the MINIMUM needed quantities with something else.
I am guessing you are referring to the Multitank 2, which requires a minimum of 270ml of liquid. There is no reason or issue with running more then that. In fact you can run (in rotation on a lift, and about double that if you are using inversion or a machine with a magnet and no lift) more then 600ml in that tank. The only maximum to be considered is the max load on the processors motor. Each model has a different max level (and in most cases even those lower ones can be safely exceeded when needed), but that usually is reached at larger tank capacities, like a 6 reel tank.
As for Expert drums, you can have upwards of 2 liters in the tank in rotation, on a lift. The minimum stated quantity is 210ml, which while is enough to wet all the films, is not enough even when using stock D76 if you have 10 sheets loaded.
Those minimum amounts listed on the tanks usually refer color processing, and denote the minimum amount of liquid needed to wet and submerge all material loaded in the tank.