I do this calculation a bunch of different ways, but I am currently converting my dashboard to 10. It takes on hand and receipts and then uses PartDtl for all requirements and POs to build a time phase of all material on the job being calculated into a UD table and then checks by either lead time if not PO has been placed yet or by actual POs to see if any material has a negative status by the material due date.
I will post it once it is working in 10 and you can see if it fits what you need.