I have relatively little experience, but here's something to try, noting the copy from Word help on fields, below.

- (minus sign) This format item adds a minus sign to a negative result or adds a space if the result is positive or 0 (zero). For example,
{ = 10 - 90 \# -## } displays -80.

Try this: select everything on your title page and check the status bar at the bottom of your doc for the number of words. (If it's not showing, right-click the bar and select Word count.) Then, simply insert this number after a minus sign in your Numwords field.

Does this solve your problem?