Replenishing QOH - Subtracting units from one Table based on entry into another Table

I was reading this post about Subtracting units from one Table based on entry into another Table regarding Inventory and Orders and getting the QOH formula field calculated.  My question is if I replenish the starting inventory, e.g., 100, how can I reset the QOH to be the same as that as the starting inventory?  Currently the QOH is calculated as the difference between the starting inventory from the parent table minus the total Orders in the child table. 

Here is the post I found that allowed me to be able to calculate the QOH:  https://community.quickbase.com/communities/community-home/digestviewer/viewthread?MessageKey=3663f84a-a355-4743-bbe7-10f5a2503b64&CommunityKey=d860b0f8-6a48-487b-b346-44c47a19a804&tab=digestviewer
Subtracting units from one Table based on entry into another Table



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Just allow me to deduct the starting inventory when an order is placed.  There is a video on youtube.com this is similar to the solution I'm looking for, except QuickBase's interface has changed as it removed the trigger portion and called it Actions instead of Automation.  The video only shows the option to copy values.  I need an option to write a formula expression to deduct the starting inventory by the order quantity to result in OOH.  That way, I can reset the QOH without worrying about the formula -numeric field.  Can this be done?  Can you provide instructions to do this?   

Here is the youtube video
Using a QuickBase Automation to Update a Field in a Related Table - YouTube
https://www.youtube.com/watch?v=fgwAiRuWnCI

Thanks in advance for your help.

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Alex Yu
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