I'm not sure you're right, Keith. My understanding is that aircraft that have belly fuel tanks as standard have the MZFM 'fudged' by the manufacturers to take account of this specifically to avoid distressing pilots with questions like 'Why do we have to take account of fuel in the Zero Fuel weight calculation?'. Some aircraft, particularly those that have been fitted in mid-life with extra fuselage tanks by somone other than the original manufacturer usually do have to take account of the centre line fuel in the ZFM calculation because the original certified limits won't have changed.