Hi Paul @Drennyn ,
As already mentioned above,
I’d only change the Units to be displayed in mm:

Do not touch the Unit Scale, it might mess up lighting, Physics and so on.
You can also safely change the Grid Overlay Scale, if you want.
Important: when you change the Clip Start, you also need to change the Clip End: Best is to divide both by the same number. So if you change the Clip Start from .01m to .001m, you should change the Clip End from 1000m to 100m.