This is typically done with relationships and summary fields
One Item has many Order Item Lines (ie the order lines)
One Item has many Purchase Order Receipt Lines (somehow you also need to increase inventory, typically via a Purchase Order Header with Many Purchase Order Lines)
One Item has Many manual adjustments (for say cycle counts or damage / loss adjustments.
Then you create a summary field of the total qty on Invoice Order Lines.
Then you create a summary field of the total received on Purchase Orders.
Then you create a summary field of the total manual adjustments.
Then on the item record you create a formula to calculate the current inventory balance.
[Total received] - [Total on orders] + [Total manual adjustments]
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Mark Shnier (YQC)
Quick Base Solution Provider
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http://QuickBaseCoach.commark.shnier@gmail.com
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