I really like this low dollar tool for getting near perfect wheel alignment. Odds are good that the marks on your swing arm are not very accurate. While it would be even nicer with a laser on it, a cheap easy way to make it a better tool is to replace the sighting rod with a long 5/32 or #21 drill bit.
Also a quick check to see if the sighting rod is straight is to simply roll it across a glass window to see if it rolls true.
Update. I see that on some computers the picture with this article doesn't show. I can e mail you a copy of the pic if you wish.
Stay greasy my friends