Ideally the Design File Working Units set in the Floor manager dgnlib, default name BB_FloorMaster.dgnlib , and the active file(s) should be the same. Note that this name can be changed and specified by configuration variable, see Floor Manager - Changing the name of BB_FloorMaster.dgnlib.
If your metric project is working in metres rather than the GB default of mm it is worth changing the working units of BB_FloorMaster.dgnlib when the project is created (and/or changing it in the project template for future projects).
There may be cases when the active file and BB_FloorMaster.dgnlib do have different units. This can lead to moments of confusion if the values shown in the Floor Master do not match the active file working units.
We have observed this can arise if the working units are changed in the active file and Floor Manager is then opened in that file. In this example the Active Units have been changed to Meters. Floor Manager was then opened and continued to show values in mm. To resolve this, close Floor Manager then reopen the file (the keyin 'newfile' can be the quickest way do this).