The minimum drop of the MF ceiling grid must allow for the installation of the grid components. The Primary channel is 44mm deep, ceiling channel 25mm deep.  You will also need to factor in the hanger which needs enough space to comfortably and safely install. This will result in an overall depth of around 100mm.

Depending on specification you may need to also factor in acoustic hangers which can increase the length of this item. The grid depth itself will be 69mm based on the primary and ceiling channel which needs to remain clear from services with the primary channels at 1200mm max centres and the ceiling channels at 400mm centres in a typical installation.