I haven't tried it, but wonder if you can add a scrap steel item with a negative qty as a component. 

I did a similar thing once with chemical assemblies.  You can add the same assembly component more than once.  So you could have the steel component of 8000kg, then add it again as wastage of 34kg.

Hopefully it works as a percentage.  Therefore (assuming unit of KG) then wastage could be a proportional quantity.  For example, if you assembly quantity is 1000KG of steel, then wastage would be 4.25KG.

Something like that.