Natural seawater around most reefs in the world is 1.0265 or 35 ppt. There is no reason it should be kept at anything other than that.
An ATC (automatic temp compensation) refractometer calibrated with a calibration fluid will read the salinity as it is.
Ron Shimek's Website...Critters
"Coral reefs are generally located in areas that have salinities in the range of
35 ppt to 38 ppt. Most of our corals, and the associated fauna including fishes, will live best at those conditions (Weber and White 1976). Most organisms, even osmoconformers, can survive for brief periods in salinities well outside their normal range. But if maintained for longer period outside of that range they will be stressed and eventually will become so damaged that they will die even if returned to their normal salinity.
Higher salinity is slightly more tolerable to these animals than is lower salinity, and adult animals are more able to withstand the extremes than are the juveniles or larvae."
"The bottom line for salinities is simple. There is simply no reason at all to maintain the salinities of our systems below normal reef conditions. All reef inhabitants will suffer damage from prolonged exposure to lowered salinities. Invertebrates kept at low salinities often die within a few days to a few months. Given that corals, sea anemones, sponges and some other invertebrates have no old age or senescence (or to put it another way, they are immortal), low salinities result in a quick death. Some mollusks, crustaceans, and most fish kept at low salinities die of kidney failure; it just takes them longer. A fish which dies in a couple of years in a hyposaline aquarium may have had the potential to live more than 20 years had the salinity been appropriate."