To get this to work you first have to create an equation (see below sample equation) that you can call up in the Custom Property box. I have just taken the surface area and divided it by 1000^2 which converts the mm^2 to m^2.
After creating the equation it is available to select in the Custom Property drop down
This would be the best way of doing it without changing the units of the part. You can then link this Custom Property in the Drawing.
Hope this solves your problem Mark
Thanks a lot that was exactly the answer I needed.