in your opinions what should a sg of salt be for a fowlr

Same as reef. 1.025-1.026. The little bit of salt saved at 1.019-1.023 is not enough to bother with.

Some say it's easier on the fish's osmotic regulation at a lower SG level but they have done pretty well for millions of years at natural sea water levels.

Also, the levels of buffering capacity at NSW level is just not there at 1.019-1.021sg.

Lastly, the skimmer is more effective at NSW than at below NSW.