Question

Here an example http://jsfiddle.net/EhLsT/

$(window).scroll(function () {
  if ($(window).scrollTop() > $("#header").offset().top) {
    $("#floating").show();
  }
  else {
    $("#floating").hide();
  }
});

I want to show my #floating after 450px scrollTop

Answer 1

I not sure if a understand right your question but il you wan to display #floating after 450px try this :

$(window).scroll(function () {
  if ($(window).scrollTop() > $("#header").height()) {
    $("#floating").show();
  }
  else {
    $("#floating").hide();
  }
});

Simply change $("#header").offset().top to $("#header").height()