missed that in the roll... other than what they describe here
Map scales and scale properties—ArcGIS Pro | ArcGIS for Desktop
and it seems your abbreviations are correct
- The following unit abbreviations can be used: in, pt, ft, yd, mi, nm, mm, cm, m, km, dd, dm.
but this comment seems a bit strange that it can't be saved
Recently entered scales are saved as part of the scale list, but are only saved for the active session. To save these values, you must manually reenter them using the Scale Properties dialog box and add them to the current default list of scales. Click Customize to access scale properties.
But did you try the next step to use the <current> option?
If that page doesn't cover everything, I would inquire why it it trying to upscale from feet to miles, it is almost like it want to limit the number of numbers in the representation, which shouldn't happen