Rolling Stock Levels for Assembly Component Levels

Using customer orders for an assembly, I need to keep a running stock total of assembly components.

For this example I will just work on the magnets.

Today 2 orders for Big Box, order number 2032 and 2033, there are 5 magnets per assembly so I need to deduct 15 magnets from Magnet, Total Quantity, for order 2032 and 5 magnets for order 2033, what formula do I need to use to maintain a "Total Quantity" of components

Tried to look for through other threads but couldn't find one for assemblies