Thanks for posting!
The calculated result you gave it is being interpreted as a true or false statement. The way it's currently set up, you're essentially asking FileMaker to verify that this field is equal to 0. If it is, a 1, or true, is returned. Otherwise, a 0, or false, is returned.
So to resolve this, just remove the field name and the equals sign from the calculated result. It should end up looking like this:
Replace Field Contents [No dialog;Table name::Target Field;0]
This will replace the field contents with 0 regardless of the current value. Please let me know if you'd like any clarification on the above.
Many thanks for your prompt response. Followed your advice, and it works like a dream.
Thanks once again,