I cannot get thePrometheus window to show centered on the screen, if i use the same code (excluding the behvavious settings) then the "standard" window will start defaulted in the center of the screen as documented.
The Prometheus window does not seem to honour the offsetelementId setting either. So for now I have gone back the standard ASP.Net Rad Window.
update: It turns out that this is caused by not having the doctype specified in the page hosting the window control. to get any positioning you need to have
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
on the page. But for me this is a no go as it destroys all the layout for the existing application.