Alex...
<<Not many aircraft count centreline fuel to the ZFM. The TriStar is the only one I can recall, apart from tankers>>
Not in normal circs - I agree. However, when you get to "abnormals" it does - certainly our 757s, with all the TWA800 fuel pump problems - meant that if we got unusable C Tank fuel, with anything other than full wings, than we had to check that ZFW + C Tank Fuel was < MZFW.
NoD