How to rotate an element with jquery on scroll percentage?

How to rotate an element with jquery on scroll percentage?

Well, I got two questions about this http://tinyurl.com/ncptoy6 :
How can I get the fixed circle element to be in the middle on any screen
width?
How can I get this circle rotating when scrolling?
I got to explain question two a little better, because I want the circle
to rotate 360* when I scroll down 100%. (example: if my site is 10.000px
high, and I scrolled down 5.000 pixels, I want the circle to be rotated
180*)
Thanks in advance!