Not sure that I fully understand the question but...
It sounds like you are trying to create a new custom attribute that is auto-numbered using some reference from the instance that the attribute is on?
If that is the case I would question WHY you think you need to do that - since an auto-numbered sequence would be unique enough anyway - I don't think you are "doing anything wrong", but you are just trying to do something that doesn't really make sense.
But if its just for display purposes within the instance itself, perhaps you could use a calculated-attribute which could then combine the auto-numbered sequence with some other instance attribute.