Editable data based on other fields

I have an "Inventory" file that requires a indexable, modifiable Job# field that can remain in the tab order.

The "Inventory" file also contains PO# and WO# fields.

- The PO# is valid if and only if the record was created via a portal in the "Purchase_Order" file which has its own Job#.

- The WO# in the "Work_Order" file also has its own Job#. The WO# may or may not be entered in the "Inventory" file.

Possible area to receive data (in order of importance):

(1) Job# "Inventory"

(2) Job# "Work_Order" (via a relationshop based on WO#)

(3) Job# "Purchase_Order" (via a relationship based on PO#)

(there will always be one of the values)

When I create a new "Inventory" record via the portal in the "Purchase_Order" file (with a Job#) I want to see the Job# of the "Purchase_Order" until I enter a WO# or Job# into the "Inventory" file. I should then see the new Job# based on WO#'s related Job# or the Job# entered in the "Inventory" file.

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I have explored using a calculated value and Ray's AutoIndexing method to set the value with no success...yet.