I've got 2 feet of snow blocking the way up hill to my back shed where my ring shank is stored and not in the mood to shovel it yet. So I can't try it out yet on a cold crucible. But I did take a look at some of Ironman's videos, and found a segment where he was pouring an A6 crucible with a ring shank.
This frame from a video shows that his ring is made of flat bar, not round (like mine was) and he has a pretty steep pour going at this point. In the video he actually shakes the last bit out of the crucible.
It looks like the ring is located at about the half way point up the crucible, not two thirds. In this photo, the plinth is stuck to the crucible, which obviously mitigates against the crucible slipping forward. But the crucible would probably have been used for pours before when the plinth was separate.
I'm not drawing any conclusions here about position, but I do believe that a flat bar is better than round re. the slipping problem. If the ring edges are square I think it would naturally be harder for the crucible to slide -- more of a tooth, and less of a pivot than the inside edge of the round bar.
