Setting the value of field in a portal row with a value taken from a field in a previous row?

[Paradigm]

I want to populate a field of a newly created record in a portal row with the value of the field in the record right above it (with a secondary key). I'm trying (wanting) to do this via an Auto-enterd Calculated Value and I've got my FileMaker 9 Developer Reference cracked open but I just can't figure out a way to make it happen.

In the past I've done this with a script that creates a new record in the portal but I in this case I need to have it so that writing in the portal window creates the new portal row record.

[comment]

You need a self-join relationship of the child table, e.g.

Child::ParentID = Child 2::ParentID

AND

Child::ChildID > Child 2::ChildID

Sort the related records on Child 2 side by ChildID, descending, and auto-enter data from Child 2.

This assumes your existing portal/relationship are not sorted by another order.

[Paradigm]

Yes, ... a self-join relationship. I think that should work. The records are sorted by another field in addition to the one whose value I want to inherit but that sort is the secondary one.

I'll give this a whirl. Thanks.