It is not calculated so much as chosen. MLW is defined in the initial specification and the aircraft has to be designed to meet all applicable requirements at that weight. Landing gear loads will figure highly in that assessment - airframe vibrations are usually high airspeed problems.
The choice of MLW will depend on how one expects the machine to be used.
On a dedicated long range usage for example MLW mignt be something like MZFW plus an arbitrary but sensible fuel allowance - say total fuel reserves uploaded for a flight starting at MTOW but none of them consumed.
If the aircraft is expected to fly a series of short sectors through the day without refuelling then MLW could be close to MTOW to cover the first segment.
On something like a Cessna where the total fuel is a relatively small percentage of the total weight then MLW and MTOW may have the same value.
These are just examples to illustrate the thinking.
Last edited by Owain Glyndwr; 22nd Sep 2016 at 15:51.
Reason: various typos and additional comments