It looks like at the moment you are running at about 840Mhash/s.  So average hashrate is the last ten rounds combined.  So if you have been using that same hashrate since you started mining it would been that about 6 of those 10 rounds you have been mining 840 Mhash/s.  Note this is only a guestimate.  It's like saying I am currently running at 100Mhash/s and have always done that.  But I only did it for five of the last 10 rounds so my average hashrate would be 50 Mhash/s.  If you stay steady at the 840 for ten rounds then it should show an average hashrate of 840.