You can assign a server side script to do a find on the value and then loop through and send the mails based on the returned list of values. You can schedule the script to run at timed intervals.
That calculation is based on other fields - possibly a field in Transaction, or an InventoryMovement table? When you make an entry into THAT field have a script that commits the record and then looks at the inventory calculation, sending the email if needed.
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