Purchasing item that reduces level of available stock

Hi,

We have a range of products (cables) that are made bespoke to our industry.
We have purchased a load of copper cores in varying thickness.
We then have a BP who manufacturers these cables using our copper cores as requested.
How can I do a PO for the cables that will automatically reduce the stock of copper used?

E.G.

to get 100 Meters of cable A
We use 400 Meters of core 1 and 400 meters of core 2.
So when we raise a PO or a GR PO, currently we need to minus the quadruple value of each core used, which makes human error an issue as we have many cables using all sorts of core types and lengths. Getting the correct values happens less than getting it wrong.

I don't want to go down a production route, as we don't produce them and will still need to raise a PO for the cables to be manufactured.

Thanks