I don't think getting involved with trig is necessary. But, I'd like to see you remake the topic with some more information on other aspects of drag-dropping, eg:
startDrag(target,[lock, left, top, right, bottom])
which can be used to allow dragging only along one axis (useful for things like volume sliders, etc). Check out startDrag in the AS Dictionary, make some sample files until you know exactly how it works (which shouldn't take too long), and THEN write the topic.
I put it on the list because it's an oft-asked question, I just think it was a little too simplistic. Something that covers a bit more of the potential of the command would be better....