Hi Kevin,
That's a great question! Basically you're trying to perform a subtraction on two different field types. In your case is numeric and duration. So in order to make it successful you need to confer FieldB to numeric value (covering FieldA doesn't make sense since the final output is numeric).
The formula should be something like this (FieldA-FieldB/Hours(1))/FieldB/Hours(1)
Please let me now if that worked out for you!
Regards,
Tamir