Using an On File Close... script, I store the layout the user was on when exiting. You could do a similar thing and store the Zoom Level. I use a Sessions table and every time a user opens the file, a script runs checking the last record that user created for their end state and creating a new record for the current session. That may be overkill (you really only need one record per user and store the info in that).
And Get ( WindowZoomLevel ) can access the current window's zoom level.