Scale List Edit
One of my all-time favorite AutoCAD toolbars is the Viewports toolbar. This little gem lets you do all manner of Viewport operations: make Viewports, make non-rectangular Viewports, clip Viewports, and quickly set or view the Zoom Scale factor of a Viewport.
When choosing the Viewport scale you are not limited to the scales listed in the drop down. You can type in your own desired scale in the edit box. You can also edit the list of available scales with the SCALELISTEDIT command.
The Edit Scale List dialog lets you add your own scales to the list. You can move scales up and down in the list, delete entries, put your favorites first, or put those pesky metric scales at the end of the list. (Who uses metric anyway?)
Don't worry about messing up the list of scales; you can always use the Reset button to return this list to factory defaults. Changes made to this list of scales also affect the scales available in the Plot dialog.