How To Get OnClick Popup Centered On Site

I have a div popup on my test site and i'm having trouble centering it. For some reason (which probably has everything to do with the code) the pop up shows in the middle of the page, as opposed to the middle of the screen. Forcing people to scroll down to see it. You can check it out here: Test page. Is there any way I can code it to where it always shows in a visible
position?

Here's my javascript code:
function toggle(div_id) {
var el = document.getElementById(div_id);
if ( el.style.display == 'none' ) { el.style.display = 'block';}
else {el.style.display = 'none';}
} function blanket_size(popUpDivVar) {
if (typeof window.innerWidth != 'undefined') {
viewportheight = window.innerHeight;
} else {
viewportheight = document.documentElement.clientHeight; .....

And here's the html:
Code:
<div id="blanket" style="display:none;"></div>
<div id="popUpDiv" style="display:none;">
<a href="#" onclick="popup('popUpDiv')">Click Me To Close</a>
test
</div>
<a href="#" onclick="popup('popUpDiv')">Click Here To Open The Pop Up</a>