Vertical Centered on Screen in Facebook Canvas Iframe

When I use fancybox in a iframe embedded in a facebook canvas page, the vertical centering is 50% of the iframe height instead of the browser window height… as a result, if the iframe/page is long then the fancybox may be popping up beyond the visible browser window area and the user needs to scroll down in order to see it. What’s the best solution for this?

http://wordpress.org/extend/plugins/easy-fancybox/