Subbu,
As I said earlier unless you have % Impurity which you want to have less than 5% should be assigned as dummy component in FG BoM along with SFG with least minimum value so that when you create a order in component you can see % Impurity for which you can calculate based on MQC Formulae and otherwise you can directly assign the % impurity in order itself.
Try this way and See how it works.
BR
KK