QPainter ,QPainterPath...and cubicTo()

Hi,
I want to make the green circle move through the big grey circle as if it is a progress bar...
i am not at all sure how to accomplish this,for my part i think it would be somehow through QPainterPath,its what i think....
also i cannot understand the CubicTo() function of QpainterPath..the diagram in the document http://doc.trolltech.com/4.4/qpainterpath.html#cubicTo
makes it even more confusing for me..any help ..atleast on the first question;)

what i have learned is that painterPath()'s pointAtPercent() function should solve my problem ..but i can help myself with this...any suggestion in this regard would be helpful