Time Phase balance field

I am on 9.05.702a and trying to get the time phase details to SO backlog. I would like to display "projected balance" on the SO-rel due date. 
It should be on hand quantity + supply - demand. How would we put this into running total field?
Can it even be done with baq calculated field or I need to create a view and go with the external baq?
Please advise
Thank you for your time!