How much sea salt should I add to my molly tank?

I have a molly tank in the begining stages of being set up (I have the tank, the substrate, the fake vegetation, and the water added). Right now the tank is just running with no fish in it, I'm letting it cycle first. I've heard that adding sea salt to the molly tank is beneficial to these fish, but a lot of variation in the amount recommended. I've heard everything from 1-2 tsp per gallon to "half the specific gravity of sea water" I realize salt isn't required, but I'm interested in maintaining a brackish water tank, so I'd really like some help. If half the specific gravity of sea water is recommended, what does that calculate out to per gallon?