scroll up and down a div on button click using jquery

Question

I am trying to add a feature to scroll up and down a div based on button click. I was able to do the scroll down part easily, but got stuck wit the scroll up part and one more concern was a scenario, which I will explain,

Click on "Go Down" button.

and if I manually scroll down dragging down the scroll bar.

and now if I click on Go Down button again, the scroll bar will go to the previous position, as the variable assigned with the value for scrolling has an old value insteading of detecting current position of scroler.. I will add a jsfiddle link to show my work and also paste the code. What could I be doing wrong wit the scroll up option too!!

http://jsfiddle.net/xEFq5/7/

var scrolled=0;

$(document).ready(function(){


$("#downClick").on("click" ,function(){
    scrolled=scrolled+300;

    $(".cover").animate({
        scrollTop:  scrolled
    });

});


$("#upClick").on("click" ,function(){
    scrolled=scrolled-300;

    $(".cover").animate({
        scrollBottom:  scrolled
    });

});


$(".clearValue").on("click" ,function(){
    scrolled=0;
});


});


<div class='header'><button id='upClick'>Go Up</button> <button id='downClick'>Go Down</button><button class='clearValue'>Clear Value</button> </div>


 <div class='cover'><div class='rightSection'></div></div>

also is there a good plugin which has this functionality??