Search this forum, for "inventory log" and you'll find several threads on what you describe.
You need one more field: cBalance as Qty Purchased - Qty Used.
Then you can define your Running Balance as a summary field that computes a running balance of cBalance.
Instead of continually updating the same record, create a new record for each inventory update. This gives you a ledger style record of how your inventory levels are changing over time. You can sort your records by Product to group them so you can see the Quantity on hand for each product as the running balance of the most recent entry.
I am almost there. The ledger format work well for me but I need the cbalance and sbalance to reflcect only one item id. Right now they are adding and subtracting all items together. I am using filemaker 6.
You said the sBalance should be (Summary, running total of cBalance, restart totals when grouped by ItemID). I don't know how to get it to restart totals when grouped by itemid.
Hmmm, I'm not sure when they added the "restart totals" feature. It may not be available in filemaker 6.
You can still get your sub-totals by part, but it takes a bit more work. Create a summary report and place your summary fields in a sub-summary part specifying a "sorted by" field that will group your records by item. That sub-summary part will then display your sub-totals when you print or preview your report.
Here's a link to a simple tutorial on setting up summary reports that you may find useful if you've never worked with summary reports before:
Creating Filemaker Pro summary reports--Tutorial