Most SWGs (not all) display the conductivity of the water as measured through the cell (in units of equivalent salt level), which is exactly the number the SWG actually cares about, the number that determines if the unit will work or not. "Calibrating" that number to an external measurement makes no sense, as external measurements have nothing to do with how the SWG is going to perform. If the conductivity is too low (or high), the SWG will not work, regardless of what the actual salt level is.
Some SWGs measure salt separately from the cell, and for those units the calibration of the salt sensor is interesting.