here is a quote from Randall holmes-farley from an article on refractometers
The salinity on natural reefs has been discussed in a previous article. Based on such information, my recommendation is to maintain salinity at a natural level of about 35 ppt (abbreviated as ‰ and also as PSU, practical salinity units). If the aquarium's organisms are from brackish environments with lower salinity, or from the Red Sea with higher salinity, selecting something other than 35 ppt may make good sense. Otherwise, I suggest targeting a salinity of 35 ppt (specific gravity = 1.0264; conductivity = 53 mS/cm; refractive index = 1.33940).
Recommendations aside, high quality reef aquaria exist with a fairly wide range of salinity. Many highly successful reef aquaria have salinity in the range of 32-36 ppt, or specific gravity in the range of 1.024 to 1.027.