Metric Designs

No variable however you can accomplish this in a couple different ways.

1. Use a startup script that will set the drawing units, along with other items you find helps start out a new drawing such as setting colours etc.

You can then start Allegro using the -s  (script command) e.g.:

allegro design -s startup_script  (Where s"tartup_script" is the name of the script you recorded)

OR

Using the variable "script_startup" found in the Ui category of the Preferences editor specify the script. See the Preferences editor for a description and use of the variable.

2. Import a techfile that contains the units that you need. You can also use the techfile facility to setup the stackup.

o Create a new drawing with the units and stackup desired. You can create as many as you like for different units, stackup, constraints etc.
o File > Export > Techfile...; Add a name and select "Export"

o Start a new drawing
o File > Import > techfile...; browse to the techfile you need and select "Import".

Hope this helps

Another way to do it, and it's my methodology, is to create "start" boards that I use to start all my designs. It has the Units established; the fab and assembly drawing formats in; the notes for all drawing formats; and anything else I like to personalize. This works for me.
Then I have scripts to set layer stackups, and different tech files per technologies.
Good day.
Mitch