1 tablespoon per gallon = 4.5 ppm. What you want is at least 3 ppm. The exact measurement is 2 tsp per 5 gallon, but since all the ich might not be eradicated if it's slightly below 3 ppm, people round up a bit for obvious reasons. Since most fish can handle 5 ppm if it's raised slowly, there's a tendency to use the higher amount.
Your goal is to keep the level at 5 ppm, but not under no case should it be lower than 3 ppm. Evaporation raises it; so adding back water lowers it back to the target level.
Therefore, you are NOT supposed to add salt "regularly." Once you hit the target level, you should only add salt after you remove water, and in that case only in proportion to the amount of water you removed and replaced. E.g., if you take out 20 gallons, when you add back 20 gallons, add 4 tablespoons of salt, etc. If instead you top up the tank due to evaporation, you don't add salt.