Calculate Quantity Remaining

Summary:

We have requirement to get the difference between the quantity stored in two Bos Bo1 and BO2.

The result of the difference should be stored as Quantity Remaining field in BO1.
BO1.Quantity_Remaining= Bo1.Quantity -Bo2.Quantity_Used

The Quantity_Remaning should get recalculated and should be available to be used for the next entry

We tried object function but seems to be not working.

Please advise any other approach. with a sample App or a script as reference.

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