Hi Philip, also an different way to model something like that would be like this:

Get a simple surface going first like this:






Now go to the side view and draw in a profile curve that intersects the simple surface, and mirror it to produce this:



Now you can use those curves to slice up the original surface, select the ring, run Construct > Boolean > Diff, and select those 2 curves the cutting objects. That will slice up the ring and leave these pieces behind:



Delete the pieces you don't want:




You may find it easier to try different shapes quickly by being able to draw in the profiles of the cuts like this, and then this lets you focus on just producing a more simplified surface with the initial sweep.

- Michael