Hope you will be knowing the Consumed quanity when you receive the FG/SFG Material. In that case in the Second line of Purchase order under delivery address input the Subcontract Vendor code and first perform GR for Second line.
Here once the document is posted, the stock will be updated at Vendor end.
Then you perform GR for the Parent Material code.
By the way you have the possibility of changing the component / scrap quanitity during Subcontracting GR. Give a try in Development server and let us know your findings.