Scrap Generation in Subcontracting Urgent !!

Hi Friends

In my scenario we send sheets which is raw material to vendor site and get some semifinish item. Here problem is with scap suppose i send 100 kg of raw material and 80 kg of material is consumed to manufacture semifinish so 20 kg of scrap left.And i get confirmation of scrap generation only when i receive my semifinish from vendor.Now i have question how to track scrap generation as i am not sure what quantity of scap will generate at the time of raising PO. So whats the possible solution to map this process.

Regards

Vivek