I did mine where it was slightly above the window/marker. Never had a problem since. Like Diesel said, when adjusting the tensioner you can see it tighten and loosen. There is quite a bit or wiggle room on tension even using the window.
Oh and on another note you can have the pointer on the other side of the pulley and it will still work. It moves in a arc in it's movement and you can actually turn it to the other side. See where it's at now then try adjusting a lil bit each way till you are satisfied or till the belt stops walking. Also don't forget a little bit of walking is normal at first due to the belt not being worn in, but it should straiten out after a few minutes of running.