I am using this function which works ok for one screen size. 

jQuery(window).scroll(function(){

    if (jQuery(this).scrollTop() > 1400) {

       jQuery('.conmenic').addClass('menu-invert');

    } else {

       jQuery('.conmenic').removeClass('menu-invert');

    }

});

The problem is that I have a full screen background div which is set in my css as follows:

#fullimg {height: 100vh;}

I need to somehow calculate for all screen sizes that the user has fully scrolled after the #fullimg before adding/removing the new class 'menu-invert' as seen above. 

any help is much appreciated.