Sidebar Position on Different Resolutions

Sidebar on website is displaying in different positions on different monitors.

I have had some changes made to site (addition of JS “floating” sidebar) and since then issue has been present.

On my monitor (1920×1200) the sidebar appears far to the right, if I reduce the window size it goes back to correct position, from other monitor resolutions I can test with quickly is different positions.

Far as I can figure this is the code:

#sidebar {
float:right;
width:280px;
padding:5px 0 0;
position: fixed;
right: 13%;
top: 258px;

I have tried changing the % to test but just gives different results again on different size windows/resolutions.

http://computethis.com.au/

Any help would be appreciated.