Its not as simple as just setting the x/y there are a number of things to take into account like the screen edges, size of tip etc. etc.
It turns out that ToolTipManager has a function to position the tooltip that takes all of this into account perfectly. Unfortunately, as is proving to too often be the case in Flex for me, it was declared private so I cant access it. The solution, was to duplicate the code in my own class, trivial to do but duplicating adobe code was something I am trying very hard to avoid in Flex as a point of principle.
