I wouldn't include a Checked Out field in the Inventory table, but calculate it in a query.
For example, create a query based on Inventory, and add a column
Checked Out: DSum("Quantity","Checkout","[Tracking #]=" & [Tracking #])
This assumes that Tracking # is a number field. If it is a text field, use the following:
Checked Out: DSum("Quantity","Checkout","[Tracking #]='" & [Tracking #] & "'")
Regards, Hans Vogelaar (http://www.eileenslounge.com)