You seem to have two different issues here. A conditional format expression is used to change the fill color, the text color or a number of other font formatting options.
The question mark, on the other hand is FileMaker's way of telling you that it can't display the field's value correctly in the layout's field object. This could be due to a math error such as division by zero, but then the question mark is still there when you click into the field. If you can click into the field and see the correct value appear in place of the question mark, you need only make your field wider so that it is wide enough to display the formatted data correctly.
It figures that my first foray into the forum would have an answer this embarrassingly simple!
Thanks so much for solving this for me, I will be more diligent with my troubleshooting before posting next time.
No need to apologize! You may feel foolish when the answer turns out to be simple, but think how foolish it would have been to remain frustrated on this issue. And while the answer is simple when you understand the cause, it's not a question that would have been easy to find by searching help or the knowledge base so asking here makes perfect sense.