Matt,
Thanks for your response. I tried to use the Rule you suggested, but to no avail. The name of the textbox field is ''ActualQty'', it has a default value of ''0'' generated by the database (it is not actually NULL) and my Rule looks like this:
Rule 1
Condition: ActualQty = 0
Action:
Set a fields value: . = OrderQty + StoreAdj + CommAdj ...