Cells that have been contaminated with foreign material generally read low of the actual conductivity. Your zero (in air) conductivity reading is generally unaffected.
The conductivity error due to fouling will generally be proportional to the conductivity value. Conductivity is corrected not as an offset but as a ratio (multiplicative) error compared to a reference.
Salinity is a derivative measurement of temperature, conductivity, and pressure, and should be corrected by adjusting the component measurements. Generally speaking, an error in the conductivity measurement will correlate to a directly proportional error in the salinity measurement.