To calculate the distance from the top and save it into a variable you can use the following code:
$(document).ready(function(){
var elementOffset = $('your-element').offset().top;
});
Another useful technique is to determine the current distance from the top for the element while the visitor is scrolling up/down the page. To do this you would have to catch the current page offset from the top first.
Here is the code:
$(document).ready(function(){
$(window).bind('scroll', function() {
var scrollTop = $(window).scrollTop();
var elementOffset = $('your-element').offset().top;
var currentElementOffset = (elementOffset - scrollTop);
});
});