Prev/Next Links from Absolute to Fixed on Scroll

Hi everyone.

I got for my website’s #navigation that turns the bar from “position absolute” to “position: fixed” in my css file after x amount of scrolling

<SCRIPT type=”text/javascript”>
/* Handles the page being scrolled by ensuring the navigation is always in
* view.
*/
function handleScroll(){

// check that this is a relatively modern browser
if (window.XMLHttpRequest){

// determine the distance scrolled down the page
var offset = window.pageYOffset
? window.pageYOffset
: document.documentElement.scrollTop;

// set the appropriate class on the navigation
document.getElementById(‘navigation’).className =
(offset > 240 ? ‘fixed’ : ”);
}

}

// add the scroll event listener
if (window.addEventListener){
window.addEventListener(‘scroll’, handleScroll, false);
}else{
window.attachEvent(‘onscroll’, handleScroll);
}
</SCRIPT>

with css:

#navigation {
top: 240px;
position: absolute;
padding: 15px 0px;
}

#navigation.fixed {
position:fixed;
top: 0px;
background: white;
border-bottom: 1px dotted #d9d9d9;
}

This works brilliantly, but I wanna expand it and have my navigation links to previous/next post/page (#nav-single, #nav-loop, #nav-search) to be to the bottom of the window to move whilst scrolling until after 50% of window is scrolled of pixels to turn it into fixed at that position (50% from top or bottom, whichever)

ive been playing around with the code for a good hour but I just cant seem to get it to work

please help??