Instead of selecting the check box for this in Field Options, you could set up a validation calculation such as:
Self = "[ND]" or Length ( self ) = Length ( Filter ( self ; "9876543210" ) )
you can add additional characters for decimals and minus signs to the quoted text if you need them as part of a numeric entry.
Even better, use the OnObjectValidate script trigger to run a script that checks the validity of the entered data. This second option enables you to make data validation a much more dynamic, user friendly experience.
Thanks, I went for the OnObjectValidate in the end. A bit harder to set up for every field but the result is much better.