It seems the grid is fixed to decimal subdivision, at least I have found no other way.
Edit: Not quite true. You can have your dimensions read in feet and inches, as in the added sample. Grid spacing is different here, just have noticed now.
Just have edited my recent post while you have posted yours. You don't have to scale your drawing to fit the screen. What kind is your project, what scale should it be? If it is housing my sample might help.
Well, then a printing scale of about 1:12 for letter paper size would be appropriate, if you want to to show several views of the box on one sheet, or 1:6 or similar whatever is standard in imperials, for one view per sheet.
Even if you don't intend to print it is advisable to consider a scale in order to get reasonable text and arrow sizes in the dimensions.