1.015 is unnecessarily low, like Alice said. Most fish do fine at depressed salinity levels, at least in my limited experience. Sorry I can't give a scientifically proven formula to you, but I keep mine at 1.021 approximately.
Fwiw, I also like a lower temp for my FO, 76-77. The idea being to keep the metabolism down a bit, feed less and keep the water cleaner. Don't know how well it works though! Trying to keep a puffers appetite down is like trying to disprove the existence of gravity.