Jquery Scroll full screen div

Jquery Scroll full screen div

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

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

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.