You're correct, that is only the case for landing weight limited. If you are TOW limited it is even easier, just subtract your ZFW from the MTOW, add your taxi fuel and that is the max you can uplift. Whatever you don't use you've tanked!
It gets a bit more complex if you want to tank a specific amount as you have to consider the fuel burn required to carry the fuel. Also depends on why you want to tank, if its to save money by buying it cheap don't go to close to a limiting weight and this may give you heartburn with MTOW or MLW. If you want to avoid a refuel to get a faster turnaround at the next port don't cut it too close on the fuel on arrival as the time to top up may chew up most of the time your trying to save.