By virtue of the concept, "normal" is what the developers intend to be the balanced experience, the one that represents what the average player should be able to traverse but not feel bored or too easy. I come at it more about seeing how well they've executed on that normal and playing it at that difficult to experience it as intended. If that is too easy or too hard, then I change it but usually I stick with it. It doesn't make any sense to change the difficulty when I don't know what the normal is. Once I do I judge from there unless I already have it from the staff that the game should be bumped up to considered "normal".