Hi UTW,
Crikey - this is an old one you've dredged up.
Any more theories on why the figure of 3000 Kg is specified?
Probably a compromise in order to give you time for analysis and to comply with:
"FUEL MANAGEMENT
Tanks must be emptied in the following order :
center tank then wing tanks"
so as to take care of wing bending loads.
If the fuel tank is leaking, then best to use it rather than loose it, since you can't pump wing tank fuel into the centre tank and waste it like the Azores Transat.