If the nitrite is zero, you don't need to add any salt. But if it creeps up again you can add some. Just put the new nitrite reading into that calculation instead of 1.0.
Salt is not a good thing to use routinely. It harms fish that originate in water that doesn't have it - in other words it is not good for any fresh water fish. It's just that it is less harmful than nitrite, and it is only used until nitrite is gone.
No form of salt should ever be added to a fresh water aquarium except when there is nitrite in the water or the fish have whitespot (it can be used as a cure for that).
Once the need for salt is gone (zero nitrite or the whitespot is cured) the salt is removed from the water gradually by doing water changes and not adding any more salt.
Aquarium salt is just plain sodium chloride, the same stuff as table salt but without any iodide or anti caking agents.
At 1 tablespoon (15ml sponful) per 20 litres, that is a huge amount of salt. A level teaspoon (5ml spoon) of salt contains something like 6g, and the calculation requires a quarter of that in 100 litres!!!
If I remember correctly, API aquarium salt is crystals rather than a powder which does make it difficult to compare a spoonful of that to a spoonful of table salt. I would still weigh it rather than measure it with a spoon. I have kitchen scales which weigh to a gramme. For a half g I would weigh double what I need then divide that amount into two halves.
A lot of companies that make aquarium salt, and a number of old time fishkeepers do recommend adding salt to a tank on a routine basis. This is because decades ago no-one knew about the nitrogen cycle. They did not know about ammonia and nitrite, and there were no testers for them anyway. But it was discovered that adding salt to the tank stopped their fish dying - and we now know that is because it blocked nitrite from binding to the fish's blood.
We now know about nitrite and what to do if we find any in the water. But that doesn't stop old time fish keepers telling everyone they should still add salt, and it doesn't stop companies selling aquairium salt as it costs next to nothing to make and earns them money.