Steve,
The IC40 is known to not be the most reliable when testing salinity. +/- 500. There is a thermister in the Flow Switch assembly that often goes bad causing erroneous salt readings.
As Marty points out, the key is to keep the cell happy until the actual salt level gets above 4000 or so... As long as the cell is working, and not reporting an error, I don't care what the cell reports. When I have to increase the salt level above 3800 - 4000 range, to keep the cell working, then it is time to replace the Flow Switch.
Thanks,
Jim R.