All you had to do was use the "Number format:" dropdown on the Formula
dialog.

So, the answer turns out to be a simple one.

Perhaps I didn't word my original inquiry clearly enough, as I don't
understand why it took so many replies (and more complex remedies, e.g.
'picture switches') to get to this simple solution.

I had looked for (and not found) a 'number format' option in the dropdown
menus, not having noticed it in the 'formula' dialog box. (Logically, a
'number format' needn't be tied to 'formulas' only -- i.e., it could apply to
all numeric entries.)

Thanks.