Javascript 向下滑动时向下平滑滚动

如何使页面在向下滑动的同时平滑地向下滚动指定的数量？e、 g.75%或750px

$(document).ready(function(){
  $("#block-full-content-thumb-1").click(function(){
    $("#panel-1").slideDown("slow");
  });
});

---

请提供JSFIDLE。

$(document).ready(function(){
  $("#block-full-content-thumb-1").click(function(){
    $("#panel-1").slideDown("slow");
  });
});

$(document).ready(function () {
    $('#block-full-content-thumb-1').click(function () {
        $('html, body').animate({
            scrollTop: $('#portfolio-sep').offset().top
        }, 'slow');
    });
});

$(document).ready(function(){
  $("#panel-close").click(function(){
    $("#panel-1").slideUp("slow");
  });
});

$(document).ready(function () {
    $('#panel-close').click(function () {
        $('html, body').animate({
            scrollTop: $('#portfolio-top').offset().top
        }, 'slow');
    });
});