Having just made Legend, a couple days ago, 61.00 avg (or lower) is the requirement for advancement. I was at 61.04 and shot a 30 at BP in the weekly multi, which dropped my avg to exactly 61.00 and I got advanced.
FWIW, when your avg is at 61.30 or lower, you would need to post scores of 30 for 9 and 61 for 18 to lower your avg.
I believe the ranked round requirements have been met when your avg does not go UP, regardless of your score. Whatever the number of RR is for each tier, I would not want to speculate.
Hope this helps and good luck with your quest.