natural seawater is 1.030@ 82? i think thats wrong in most places. A long time ago i read this book on keeping a saltwater tank that i really liked. the authors name is Robert Straighan. not sure about that spelling, but he said water off of Florida is 1.025@ 78 degrees.
I collect my own water here in South Florida. At 79deg. the natural water is 1.025. Calcium is 440mg. Alk is 5.5meq/l. I would say that 1.023-1.027 at 79deg. should be fine. In a reef system, I would not go with anything lower than 1.023.