need help dynamically calculating "bottom" relative to another div - using jquery?

hi all,

i'm a novice at scripting etc, but have been tying the below code:

Code:

---

$(function() {
    $.fn.scrollBottom = function() {
        return $(document).height() - this.scrollTop() - this.height();
    };

    var $el = $('#wrap');
    var $window = $(window);

    $window.bind("scroll resize", function() {
        var gap = $window.height() - $el.height() - 10;
        var visibleFoot = 1233 - $window.scrollBottom();
        var scrollTop = $window.scrollTop()

        if(scrollTop < 100 + 10){
            $el.css({
                top: (100 - scrollTop) + "px",
                bottom: "auto"
            });
        }else if (visibleFoot > gap) {
            $el.css({
                top: "auto",
                bottom: visibleFoot + "px"
            });
        } else {
            $el.css({
                top: 0,
                bottom: "auto"
            });
        }
    });
});

---

which is basically making my #wrap div stay fixed in view as the customer scrolls down my page.

It then stops scrolling at 1233 pixels from bottom as per the "var visibleFoot" in above code...

This is working great for me, all but the 1233 pixels - it can vary per page, and ideally i'd like the script to dynamically stop 30 pixels above my #reviews div.

So i guess what i would ideally need is above code to calculate how many pixels, the top of the #review div is, from the bottom of the page.

If anyone can help with this, would be great.

kind regards
James

Replace 1233 with:

Code:

---

$(document).height() - $('#review').offset().top

---