How do you know if the belt has proper tension? I see the arrow, but what section should it be in on the gauge thingy?

Dpends on which belt you're talking about, whether it'* stock length or not (which you may not know unless you bought them both personally) and your sc pulley.

If it'* the SC belt, and you have that 2.5" pulley on, get the ZZP oversized tensioner pulley. It'll get you closer to where you were stock, and help prevent belt slip.