it's too tight don't go by the gause or feel. If it walking off towards the crank lossen the belt. I don't care about the little pointer thingy. if it walks to the valve cover then tighten it a little bit.
i put two new tensioners on it and two new belts. it walks away from the motor all the pullys seem to be square to each other and if i spin the motor over backwards the belt seems to walk back to the center of the cam gears. i do have it set at the lil pointer thing.
I've been seeing and even had the same issue. Diesel is right it's got something to do with the tension of the belt. Re-tensioning usually fixes it. They also make a idler pulley that has grooves on the sides to help hold the belt that will work also.
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.
Mine isn't even visable. my belt path couldn't be any more centered. My boss set it "his way" to the tension he liked. (tight end of pointer)
I had to let him fail to shut him up. then when it was hanging off the cams I loosened it cranked the motor loosened it some more started the motor the belt walked perfectly into the center of the cams. The pointer is some where around the 1oclock or 12 oclock position. nowhere near the little windiw thing.