The use of a proportional font for data entry during setup makes it tough to enter strings wider than the screen width, as the extra characters are entered when the cursor is at the far right edge of the screen.  As the character beneath the cursor changes in width, the entire screen shifts back and forth, making it difficult to see which character is being entered.  Particularly tough when entering special characters under the 1 key.

An alternative, suggested by Dean, might be to just use a fixed width space underneath the cursor.  That may work.  It will probably fix the above shifting, but it might make editing an existing string more awkward, as it will then cause the display to shift around as you pass the cursor over characters without changing them.