To my knowledge there is not set point of what is brackish water. It seems that it is commonly accepted that brackish is inbetween a full fresh setup, and a full marine setup. To be sure, you need to get a hydrometer to read the salinity of the water. Adding Aquarium Salt does not make a tank brackish. Brackish conditions are only created useing marine salt.
Well, I'm not going to repeat the whole article. Its a good read and should really help you on your way.