When I calculate my extra fuel I do not consider the MZFW.
As you said most of the time, I consider only MLW except when the minimum required fuel is more than MTOW – MLW (here 12.6t, which represents a quite long flight) In the latter case only I consider both mlw and mtow limitation. Just be careful of overweight landing when taking the maximum extra fuel with mlw consideration, sometimes actual zfw is less than expected so optimum level will be higher and you can get shortcut…it is good for fuel saving but makes you sweating
Wondering if it’s too simplistic.