I have a salt water pool and wanted to find out about measuring the salinity of it. In the past couple years I just depended on the SWG (it has a control unit which displays salinity level) to show me how much salt I have. Is this a reliable source or should I perhaps buy some kind of a salinity test? The second question is regarding salinity level itself. My manual states I should keep the level between 3000-3500, the Pool School advises to keep it 300 to 400 above the recommended level, does that mean I should have about 3800-3900 salinity or is 3500 enough?