When reading Airbus FCOM absolute precision is so important... it's the inner tank we need to think about...
I never suggested turning off the centre tank pumps in normal flight ... but as you know they do have the capacity to top up the inner tanks and feed the engines. That is why you turn them off during the fuel leak procedure, so that you can diagnose accurately.
Your A320 inner tanks are bigger than usual! Normal capacity is about 5,400 not 6,000. Let's say you start the procedure with 4.9 T in the inner tank then the qty will be about 3.7 T after 30 minutes in the cruise.
But hopefully you will have already landed within 30 minutes (WHEN A LEAK IS CONFIRMED LAND ASAP) so you only need to turn on the center tank pumps when an inner tank reaches 3,000 kg. for a more distant diversion airfield.