Additional FG but not increase in BOM

Hi NAV Expert

I would like to ask a piece of advice for manufacturing transaction, that is producing of FG without increasing of BOM needed.

Sample: 40 FG composed of BOM 10kls. of chicken

Now, encoder found out that during production 10kls of chicken results to 42 FG.

How can we encode the transaction in the system that 42 FG will just based 10kls not just increase to plus 20 kls.

Please advise.