You need to put it relative to the document, not the screen I would say.
Here you have some cross browser compatible javascript to get the document height.
function getDocHeight() {
var D = document;
return Math.max(
Math.max(D.body.scrollHeight, D.documentElement.scrollHeight),
Math.max(D.body.offsetHeight, D.documentElement.offsetHeight),
Math.max(D.body.clientHeight, D.documentElement.clientHeight)
);
}
...or maybe have a look at this:
http://www.evolt.org/article/document_body_doctype_switching_and_more/17/30655/[
^]
Good luck!