I've tried to do it changing parameters but I can't get what I want.

My current code:

var pxlCount = 0;
jQuery(window).on('scroll', function () {
    pxlCount = jQuery(document).scrollTop()/20;
    jQuery(".blur_slider").css({"-webkit-filter": "blur("+pxlCount+"px)","-moz-filter": "blur("+pxlCount+"px)","filter": "blur("+pxlCount+"px)" })     
});

I get blurred 'blur_slider' class properly but it starts from the top of the page. I would like to start blurring scrolling from a certain point. e.g. 200px from the absolute top.