I use a white background (Edit->Application Preferences->Background: #ffffff).
I avoid using the default layer (0) create layers for the various objects in my diagram.
I try to put each group on a separate layer, or layers, to make it easy to identify when copied to other diagrams.
Unless noted my drawings are 1:1
To change units:
(Simply change the default units for the drawing does rescale the drawing elements.)
Open original drawing.
Set the units for a new drawing to the desired units (Edit->Applications Preferences->Defaults)
Start a new drawing.
Switch back to the original window and copy everything.
Paste into the new drawing. If you have dimensions shown it is easy to verify that the scaling worked.