This question stems from the Boeing Bulletin fuel usage policy without modified centre tank fuel pumps, and then people using 'out of the box' ' lateral thinking'. This allowed the centre tank to have 2300kgs in it and use wing tanks first for takeoff. There was no instruction when to switch on the wing tanks on. Airmanship says that the centre tank fuel needs to be added to the a/c ZFW if the wing tanks are not full, but that was also not included in the bulletin. There was then a further instruction that in the descent the centre tanks should be switched off if 1400kgs were present. Thus in both cases wing tank fuel was used before centre tank was empty. If centre tank fuel is added to ZFW, and the total is below ZFW limits, I see no structural problem. The 'limitation' of fuel usage is another thing, but in the event of picking up NEI, and having no de-icing facilities available, I thought part of our job was to use our technical nouse and be preventive. There is a conflict in 'limitations' and one has to choose the lesser of 2 evils.