You might want to get a hydrometer to test for specific gravity or salinity. I use a little cheap-o plastic one, cost about $6. I would not recommend going above 6 ppt = specific gravity of 1.004. That's about the same salinity as their bodies, so half that much salt will probably do just fine. Don't know how many teaspoons per gallon that would be. Just add salinity slowly and keep testing.
Also, dissolve salt crystals in some water completely first before adding it to your tank. Otherwise, salt crystals can burn your fish if it lands on them.
If you have really soft water (the fish store can test for that), you might want to use marine salt instead of freshwater aquarium salt, as it has all the elements needed to support life. Basically, all natural waterways contain a little bit of almost every element on earth, because they are dissolved during runoff from soil, rocks, etc. FW aquarium salt is just NaCl, from what I can gather. So most people on this forum consider it junk.