Log reduction of Cl content in processed vials

How do I calculate the log reduction in Cl content in spiked vials for cycle development validation? I have values of spiked and processed vials, mean value of positive samples (spiked but not washed) and mean value of negative samples (non-spiked and washed).
As an example: Mean value of Positive: 134 mg/L, Mean value of Negative: -15 mg/L and value of one spiked vial: -16 mg/L.

Thanks for all help.