Refine a math calculation function I'm developing a small website which is responsive, and I'm planning on displaying a number of items ( Demo ) on the page. As you can see in the link the items are displayed just fine, but what I'm trying to do is calculate the exact number of items that would fit inside the window ( more specific the actual space between the footer and the header ) so none of them will be shown only by half or overflown by the footer.

I got this small function working : 

Cluster.prototype.initiate_items_per_page_calculation = function() {
    return Math.floor(Math.floor(($(window).width() - ($(window).width() * 4 / 100)) / $('body').find(this.cluster_public_classes.item_wrapper).outerWidth()) * Math.floor(Math.floor($(window).height() - ($('body').find('header').outerHeight() + $('body').find('footer').outerHeight()) - ($(window).height() * 4 / 100)) / $('body').find(this.cluster_public_classes.item_wrapper).outerHeight()));
};


Just that the calculations are not that exact. For instance I have the item's width of 260px and it's height 160px ( adding a few pixels because there two more layers of background behind the visible layer that are rotated 3deg, so there are about 5 pixels more on each side ) plus the white space between them which is 15px on each side, and there are the footer and the header heights which cut from the space, so taking all of this into consideration my calculations are not that accurate anymore. So I was wondering if I could get a little bit of help with making this work perfectly no matter what the screen size is (:
The best and easiest way to do this would be to place the items in a box container with a pre-computed height and width. Then in order to find the max amount of items that can be placed in the container, you just divide the area of the box container by the area of an item.

I used this method with your previous function and got something like this.

Cluster.prototype.initiate_items_per_page_calculation = function () {
    var o = this.get_init_calculation_object(),
        areaOfHeaderAndFooter = ( o.header.w * o.header.h ) + ( o.footer.w * o.footer.h ),
        areaOfExtraPaddingToContainer = 150 * 250 * 2,
        areaOfBoxContainer = (o.screen.w * o.screen.h ) - areaOfHeaderAndFooter - areaOfExtraPaddingToContainer,
        areaOfBox = (o.item.w * o.item.h),
        containerBoxLimit = Math.floor( areaOfBoxContainer / areaOfBox );

    return containerBoxLimit;
};
Cluster.prototype.get_init_calculation_object = function () {
    var $body = $('body'), 
        $header = $body.find('header'),
        $footer = $body.find('footer'),
        $item = $body.find(this.cluster_public_classes.item_wrapper);

    return {
        footer:{
            h: $footer.outerHeight(true),
            w: $footer.outerWidth(true)
        },
        header:{
            h: $header.outerHeight(true),
            w: $header.outerWidth(true)
        },
        item:{
            h: $item.outerHeight(true),
            w: $item.outerWidth(true)
        },
        screen:{
            h: $body.outerHeight(true),
            w: $body.outerWidth(true)
        }
    };
};