Show scroll to top link based on current scroll position on a page which height is at least X times higher than viewport.
function scrollTopLink(pos) {
var dH = $(document).height(),
wH = $(window).height(),
t = $('.scroll-to-top');
if (pos > (dH*0.7) && dH > (wH * 1.5)) {
t.addClass('show');
}
if (pos < (dH*0.7)) {
t.removeClass('show');
}
}