Move div with fixed position if window size changes

Question

How can i do the following: I need a div to change it's position when the page is opened on a narrow screen or browser window size is changed. The fixed div should stay on the same vertical position when the page is scrolled up or down, but i also need it to be lets say 10px away from the largest div(a container for smaller divs, see the picture please). Now when i resize the window the fixed div comes over the div to the right.

So the question is how can i make this fixed div stay on the same position when scrolling the page vertically and make it always be 10px away from the right div? Css class for fixed is:

.fixed{
    width:40px;
    height:40px;
    position:fixed;
    bottom:100px;
    left:100px;
    text-indent:-9999px;
}

In fact i'm trying to implement a "back to top" button, so the fixed div is this button and the largest div is page's content.

Here is what i have now:

Answer 1

change your css to make it look like this:

.fixed{
    width:40px;
    height:40px;
    position:fixed;
    bottom:100px;
    left:50%;
    margin-left: -[(maincontainerwidth/2) + 50];
    text-indent:-9999px;
}