Thumbnails Scroll value for custom slider

Question

Got my hands dirty on custom image carousel. Wrote some jquery to make it work but stuck with one issue.

Issue lies when "prev" & "next" links are clicked:

• when click "next", if last visible item is reached then thumbnails container DIV will scroll to negative left.
• Total scroll value depends on the two factors
• First, if more than 5 thumbnails are remaining then DIV should scroll to (itemWidth x 5)
• Second, if remaining items are less then 5 then scroll value will be (itemWidth x remainingItems)

How to calculate the scroll value on click if "prev" & "Next" button click?

For reference : http://jsfiddle.net/ylokesh/8bHZq/2/

JavaScript Code:

var extCarousel = {
        backBtn : $('.backward'),
        fwdBtn : $('.forward'),
        thumbItems : $('.slidetabcontent a'),
        carouselItem : $('.slide'),
        currentItemIndex : $('.currentIndex'),
        totalItemsCount : $('.currentMax'),
        lastItemIndex : $('.slidetabcontent a:last').index(),
        firstItemIndex : $('.slidetabcontent a:first').index(),
        defaultThumbnailSet : 5,

        generateCarousel : function() {
            var _this = this;
            _this.thumbItems.eq('0').addClass('current');
            _this.carouselItem.hide().filter(':first').addClass('active').show().find(_this.currentItemIndex).html($('.slide.active').index() + 1);

            _this.totalItemsCount.html(_this.thumbItems.length);
            _this.controlNavigation();              
        },

        controlNavigation : function() {

            var _this = this,
                currentIndex, lastItem, firstItem;

            // Back Button
            _this.backBtn.on('click', function(e){
                e.preventDefault();
                currentIndex = $('.slide.active').index() - 1,
                firstItem = _this.firstItemIndex - 1;

                if(currentIndex != firstItem) {
                    _this.moveCarousel(currentIndex);
                }
            });

            // Prev Button
            _this.fwdBtn.on('click', function(e){
                e.preventDefault();
                currentIndex = $('.slide.active').index() + 1,
                lastItem = _this.lastItemIndex + 1;

                if(currentIndex != lastItem) {
                    _this.moveCarousel(currentIndex);
                }
            });

            //Thumbnail Click
            this.thumbItems.on('click', function(e) {
                e.preventDefault();
                currentIndex = $('.slide.active').index();
                _this.moveCarousel(currentIndex);
            });

        },

        moveCarousel : function(currIndex) {
            var _this = this;
            _this.carouselItem.removeClass('active').fadeOut('fast').eq(currIndex).addClass('active').fadeIn('fast');
            _this.activateThumbnail(currIndex);
        },

        activateThumbnail : function(currIndex) {
            var _this = this;
            _this.thumbItems.removeClass('current').eq(currIndex).addClass('current');
            _this.setCurrentItemIndex(currIndex + 1);

            if(currIndex == _this.defaultThumbnailSet) {
                _this.slideThumbnails();
            }

        },

        slideThumbnails : function() {
            var _this = this;
            var remainingThumbnails = _this.thumbItems.length - _this.defaultThumbnailSet;
            var itemWidth = _this.thumbItems.width() + parseInt(_this.thumbItems.css('padding').split('px')[0]*2);
            if(remainingThumbnails > 5) {
                $('.slidetabcontent').animate({
                    'left' : -(itemWidth*5)
                }, 'slow');
            } else {
                $('.slidetabcontent').animate({
                    'left' : -(itemWidth*remainingThumbnails)
                },'slow');
            }
        },

        setCurrentItemIndex : function(currIndex) {
            var _this = this;
            _this.currentItemIndex.html(currIndex);
        },

        init : function() {
            var _this = this;
            _this.generateCarousel();
        }
    };
    $(document).ready(function(){
        var extensionCarousel = extCarousel.init();
    })