Customize popup placement on screen

I have a question:

Up till now, I’ve been able to handle creating a popup window using JavaScript and Flash. However, between all the tuts I’ve done, I can only get the popup to open either at the top-left corner of the screen, or exactily in the center.

How does one customize specific placement of the popup? (Say off-center a bit or in from the top left corner by 50 pixels?)