Metric in Drawing as Default (can't set it)

I have managed to set my work areas as metric except in Drawing.

I go to drawing properties and set units to millimeters, which it correctly does, but the box for "set this to default" is greyed out.

I can change other properties and the box is clear and does what it's supposed to, but???

Am I overlooking something??

Thanks

Preben

 

Kind of makes sense, when you know it

Thanks!!

Preben