The string tension is inversely proportionate to sweet spot , if you will increase the tension then sweet spot will reduce and more power will generated , so for beginner it is always advice able to have low low tension because it will give them better sweet spot and more use of arm will come into play , as they are not perfectionist so will play better while on other had professional player will have high string tension.
For string tension, you have also considered the racket strength also because over and above of that level it can break the badminton racket. You have check before buying the racket what is the maximum.
Following is the grid according to level of the player you are. As suggested earlier, if you are beginner you should go for low tension and if you are.