Double taxatiok relief is not a form of tax avoidance rather its a way of not punishing an individual from paying tax twice which means if you pay in country A where the income is generated, you don't have to be in country B where you are located or where you want to bring in the income. It then means that there is nothing like zero percentage of tax because for you to enjoy the double taxation relief, you must show evidence that you had initially paid.

Another thing worthy of note is that double taxation relief is usually on the basis of reciprocity I.e there is hardly a way one country would declare a particular income free and another country would tax that. Its usually a well thought out process by representatives of both countries. A further thought process needs to put in place to decide if this is an avenue worthy of exploit.