I also keep brackish tanks. I use a Instant Ocean hyrdometer. This one goes all the way down to 1.000, making it perfect for low end brackiish. I would have to do some reading up, to recommend a Specific Gravity for your fish. What brand of salt you are using and how packed down the salt is, can make a difference in the amount you use, so using the hydrometer is the safest route for brackish.
Some brackish fish, can be sensitive to quick changes in Specific gravity, so gradual changes are better for the fish and tank cycle.
I cycled my tanks with brackish water. Using freshwater established media, on low end brackish, (like you are doing) will help. Using freshwater media in high end brackish, you will have bacteria die off.
For now, I would keep up with the water changes to keep the tank as fish safe as possible. Feeding contributes to poor water quality, so stop feeding for a few days, or feed very little. Your tank, will finish it's cycle with time and patients.