You might want to set up a "ledger" for this. Think of items received as deposits and items sent out as withdrawels. Each type of item is a different 'account'.
You can set up a table with at least these fields
ItemID (link to other tables to access name, description type data)
TransType (Shipped, Received, discontinued, shrinkage, etc.)
cBal (calculation: QtyRcvd - QtyRemoved)
sTotal (Summary, running total of cBal, restart totals when sorted by ItemID)
Every time you ship out items, make a new record and log the Date, QtyRemoved, and TransType. Every time you receive or manufacture new products, make new records and log Date, QtyRecvd, and TransType.
Keep your records sorted by ItemID on a list or table view layout and you can see your current totals on hand for each item as well as how your inventory levels are changing over time.
Thanks for that PhilModJunk,
Sorry for not getting back sooner, I had a few "easy" tasks thrown at me on Friday afternoon after posting that question which I am now completing.
I have managed to implement a variation of what you suggested which seems to be working brilliantly.
I have modified things slightly in that I have two separate tables for orders and resupplies, the ordering one has details on the customers we are shipping to and the resupply one simply has a field for the date that the supplies were received and the quantities of each that we now have.
I did wind up spending the better part of this morning hunting for why the running total didn't reset with each ProductId, but that appears to have been because I was sorting it by the ProductID from the products table and not the ID in the Orders table.
Thanks again for that, made me look quite intelligent and capable, until I then pointed out that I got some help from the Filemaker Forums...