I'm looking for some help with the code for the following requirement.  This is for V11 CE.

In Vendor Bill (account.invoice) I have added a custom selection field named "coordinator_approval".  The selection options are 'Not yet approved' and 'Approved'.  The reason I have this field is so that a team member can approve an invoice BEFORE the accounting person validates it and permanently places it in the system.

Currently I have the default value for the field set to 'Not yet approved'.

What I would like is to make the default value  'Approved' IF the Vendor Bill is created from a Purchase Order, in which case the field: 'origin' (Source Doucment) would have a value in it taken from the source Purchase Order.

To summarise, the logic would be something like this:

On creation of a new Vendor Bill,

    if field:'origin' = True,

        then set field:'coordinator_approval' to 'Approved,

        otherwise set field:'coordinator_approval' to 'Not yet approved' BUT allow the field to be manually changed by a user to 'Approved' even if the field:'origin' = False.

Apologies for my homespun logic format above, but can anyone help me with how to achieve this?  Some combination of determining a default value within a computed field perhaps?