An ATO sensor error will not cause "drift" over time. If you fill it a bit too little one time due to sensor, the next time it will automatically fill a bit more. That little-less/little-more cancels out. But that is not true of water exchange. Each time the sensor is off a bit, salinity stays off by that bit. Largely because the next time it is used will be for ATO, not salt. If your sump is 2' x 2', a small 1/16" error is 0.15 gallons.