How to adjust 46 mower deck idler strap

I have been rebuilding my 46 mower deck so I removed all components off the deck including the idler strap. I don’t totally understand its purpose but it seems to limit the travel of the idler arm so it does not go too loose. However, with the spring pulling the opposite direction to keep the belt taught, it still isn’t clear to me why it is needed. I have not found any mention anywhere about this part when people have worked on their decks. So, what is the proper adjustment for the strap? See the attached photo. Thanks