Javascript: On window scrolling, do something based on % of document height

gistfile1.txt

// On window scrolling
$j(window).scroll(function(){
	// update the ruler's color based on scroll position
	var docHeight = $j(document).height();
	var scrollTop = $j(window).scrollTop();
	var currPos = scrollTop/docHeight*100;
	//console.log( currPos + "%");
	if( (currPos > 13 && currPos < 21) || currPos > 64 ) {
		//do something in between 13% and 21% and grater than 64%
	} else {
		//undo things 
	}
}); // end of scrolling