Updating Inventory for Assemblies

Hello,
I have a spreadsheet to keep track of components needed to make an assembly. I am looking to adjust the quantities of individual components based on adding or subtracting an entire assembly. Here is what I have come up with:

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The values in column B are the quantities needed for 1 assembly. The values in column G are the quantities in stock. The value of cell K4 is the number of assemblies to add or subtract from the quantities stocked (Column G).

What I have coded works, but I was wondering if there is a better way to do it using a loop or something. I only included code to process 6 items so you can see what is going on, but there are actually 30 items to process. Also, the way it works now is you input the amount of assemblies to adjust in cell K4, hit tab or enter then click the update button. I would rather have it so you click the Update button and have it prompt you for the number of assemblies to add or subtract. Thanks for any help.
Paul