Conductivity should be roughly proportional to salinity so you need to check this with a calibrated refractometer. That being said I think that it's mainly useful for trends - meaning that my tank will have a different conductivity than yours even with the same s.g.

My tank is stable at 60.5mS with SG of 1.026 with a calibrated refractometer. I noticed it creep up to 61.3 over a day or so and found that my ATO was set a bit low and SG had crept up to 1.028. Later after a 10% water change my conductivity changed to the 61 range at the same S.G.