How to initially position a draggable

How to initially position a draggable

Hi,

I have a div which I have made draggable. The div only appears once a button is clicked. Depending on how the user has scrolled the page, if the button is near the bottom of the page, the div is appearing below that. I'd like to be able to always set an iniitial start position of the div so it's in the middle of the page, similar to how it seems the dialog boxes are. Is there an easy way to do this?

Thanks,
Steve