To measure the distance between a pivot offset of an object and the surface of another object:
Setting the Display mode to Normal, X-Ray, Models Only, or Models Display Wireframe mode enables you to see objects and models, their bounding box centre (displayed as a Magenta hexagon), and their Rotate and Scale pivots.
The cursor changes to a 2D cursor ( ).
The object is highlighted, and the object's bounding box centre (displayed as a Magenta hexagon) and the Rotate and Scale pivots are displayed, enabling you to snap to the bounding box centre of the object.
The system Ruler "object" start point snaps to object's pivot offset.
The measuring distance updates as you start to move the cursor.
The cursor changes to a 3D cursor ( ).
The system Ruler "object" end point snaps to the surface of the second object. The measured distance between the pivot offset of the first object and the surface of the second object is displayed.