Well, I WAS trying to do the whole ring but it wouldn't work which is why I was trying to identify the problem by selecting smaller and smaller sets of edges. I burrowed down to find that outside curve was the issue which is why I posted it. I want to do the whole ring, don't get me wrong. :)

I guess the problem lies in how I created the outer scallop shape in the first place then? That vertical line seems to be the fun killer. Is there a way to remove it or does it all come down to how I create things in the first place? Basically, get the first steps right or it's never going to work?

Is the general idea in MoI "large sweeping actions" rather than "small, cumulative actions"?