Thanks for the info everyone on the A340 weights - that is a big help. It's rapid exit taxiways, and geometrically the aircraft can't use them for takeoff (i.e. fully laden), so the landing weight is the design weight (and it makes quite a difference).
I've taken supercarb's comment and used:
A340-600 : MTOW 368t, MLW 259t, MZFW 245t
A340-500 : MTOW 372t, MLW 243t, MZFW 230t.
Being conservative of course, I also checked it can take occasional aircraft at MTOW without
