How do I know that the quantity of salt is enough for a given amount of flour?
There is no such way, because the amount you add is arbitrary. There is no special ratio, you just add so much that people will like the taste. The problem is, different people like different amounts of salt. For any reasonable amount of salt you choose to add, it will be too much for some people and too little for others. 

If you are always baking for the same people (your immediate family, perhaps) you can find out by trial and error how much they like, or even get them accustomed to a certain amount over time, which will become their preferred amount. If you are baking for others, you just have to give up the idea that the salt will be right for everybody.