Hi,
I don't really know how you're using the tooltip, i.e. in a menu or just from a link. However, if you are trying to do a tip from a text link and you want to position it, you might try instead doing a popup menu, just set it up like a tip. Pop up menus can be positioned by images so you could use a 1x1px transparent image and set the menu to open there, even if you open the menu from a link. Pop menu sample positioned by image
http://milonic.com/menusample24.php
This is not a tooltip menu, it is a regular menu that is just being opened by mousing over a link or an image. I just thought it might give you what you need.
I'll check on the tooltip and see if you can position it by an image, I'm not aware that is possible, but I don't know everything
Ruth