In the Route the system allows you to put a scrap percentage per operation for variable scrap which is good. But I do not see where you can add a constant scrap value to the operation? For instance, we have an operation that generates 1,000 feet of constant scrap regardless of order size and an additional 2 % of variable scrap based on order size. So, if the order size is 5,000 feet the operation will calculate 1,000 feet of constant scrap and 2% of variable scrap (100 feet) for a total of 1,100 feet of scrap. If the order is for 50,000 feet, the operation will calculate 1,000 feet of constant scrap and 2% of variable scrap (1,000 feet) for a total of 2,000 feet of scrap.
I agree with your observation that there is no fixed scrap factor at the routing level. This variable scrap at the operational level is there to accommodate variable yield by operation. This can have a dramatic effect on materials needed it out the operation and also capacity needed for a given operation. I for example had a client that had a 50% yield loss at the fifth operation step. They manufactured high-voltage insulators in this step occurred after the firing portion of the process. Unfortunately the hidden flaws could not be identified until this process.
As you probably already know, you do have fixed scrap on the bomb level. This would handle the raw materials lost within the process.