Changing the Ruler preferences
You can change the settings of the Ruler in the Ruler preferences via the Settings 
Preferences menu, via the Ruler tool context menu, or via the Ruler context menu in the scene.
To access the Ruler preferences, do either of the following:
In the Viewer window, left-click the Ruler tool (
) arrow (context menu) in the Camera View Display toolbar options (
) and choose Preferences from the context menu.
Ruler tool context menu
From the menu bar, select Settings
Preferences and in the Preferences window, click the Ruler preference option.
Ruler preferences
To change the Ruler default settings via the Viewer window:
A system Ruler "object" is required in the scene.
In the Viewer window, right-click the system Ruler "object".
A context menu displays.
Choose the Path Options context menu to access the Ruler preference settings.
