3rd needle , mid range.
Do not touch, unless your a really really good engine tuner and you have your motor fully tweaked. You can then fine tune it just a tad more.
But everyone says dont touch it. Reason being from what I heard is the factories flow test each engine and set the needles accordingly. I know I wont be touching mine soon and when I do Ill be making a mark to let me know where it exactly was. So I can always reset it.