How to get "fixed" position of an element?

I want to get an element's position relative to the window (fixed position).

Here's what I've got so far:

(function ($) {
    $.fn.fixedPosition = function () {
        var offset = this.offset();
        var $doc = $(document);
        return {
            'x': offset.left - $doc.scrollLeft(),
            'y': offset.top - $doc.scrollTop()
        };
    };
})(jQuery);

$('#thumbnails img').click(function () {
    var pos = $(this).fixedPosition();
    console.log(pos);
});

But when I click a thumbnail, it appears to be off by about 10 pixels or so. i.e., it will give me negative values for y even when the top edge of the photo is about 5 pixels away from the top of my browser window.