Show calculated field in Workflow decision criteria

Hi, I am working on a requirement where I have to add workflow condition in document query/ query condition to display "invoice journal lines.Net amount > XYZ amount. where invoice journal lines Net amount is the sum of all Debit or Credit amount of a journal. any idea how I can add this field in the document query as this Net amount is not a physical field.

Implement the calculated field as a method of the workflow document class. You’ll find an example in How to: Create a Workflow Document Class (and in workflow document classes shipped with AX).