BOAC that is straight out of our OM, which is Boeing procedure based. The idea is not to let a pump run dry. The master caution logic will only trigger if both are dry, thus the procedure of switching one off so you get a master caution warning. Minimum quantities are 2300kg for takeoff, 950 for climb and cruise and 1400 for descent.
But I am always keen to learn. How do you do it? Comparing notes like this is one of the main benefits of Pprune for me.