Dear,
Component scrap excess % you plan using that BOM will be copied in Production order and in confirmation.
So component scrap will not help you. If you use it then you need to change the component quantity manually in order or in confirmation. But if you use assembly scrap at header level it will work for you but problem is that it will increase the quantity of all components.
Only work around for this is don't maintain the any scrap and run MRP on consumption base reorder point planning or safety stock for MRP.
Regards,
R.Brahmankar