Hi!
First of all, I assume that raw material and processed material (finished product) are different materials records in SAP.
If you have a processing weight loss for a particular raw material, then you need to specify it in BOM of the finished product. Open BOM of the finished product, find the raw material that has this weight loss, go to details and enter component scrap. 1mt from 20mt is 5%, so you need to enter 5%.
Next, when you create a subcontracting PO for the finished product, the system will automatically understand that for 20mt of finished product, it needs to send 21mt of raw material to the subcontractor.
Upon receipt of the finished product (20mt) from the subcontractor, the system will also automatically post goods issue of raw material (21mt) using 543 movement type.
You don’t need to use 551 movement type because the weight loss happens at the subcontractor’s site and you don’t need to keep track of it. All what is matters is that the subcontractor needs 21mt or raw material to produce 20mt of the finished product.
I hope it helps!