How to determine a false reject rate acceptability threshold?

I'm looking to determine a false reject rate acceptability threshold. The concept being if you are having too many false rejects it indicates a loss of confidence in the metal detector. Does anymore have any insight or literature into this? 

When was the unit last calibrated?  Did you change speed/density of product on conveyor, what about packaging supplier?

https://www.profoodw...-product-effect

https://www.cassel-i...-false-rejects/

https://www.foodonli...se-rejects-0001

https://www.profoodw...-product-effect

https://www.loma.com...metal-detection

https://www.elmedint...l_Detection.pdf