While using CAD there is almost a constant movement of zoom, pan, and rotate on screen.  But on occasion you want to see the geometry on screen at true scale, as if you were holding it your hand.  There is a function for setting the screen scale factor, but you won't find it on the default menus.

You must enter into the customize toolbars area.  Then find and add the function "Zoom Ratio" to the toolbar.  This function can be added in any mode - Modeling, Drafting, or NC.

Zoom Ratio has a live feedback value, so even while using zoom, pan, and rotate, it is showing you the approximate zoom factor of the screen.