So, here's my experience.
I can do a half-swirl with maybe 60-40 of the string, no problem. In fact the first part of a half-swirl is now muscle memory for me, the aeroplane, well, that's another story...
For a full swirl I noticed that the ball was always catching up with the ken, making the catch quite hard. I tried giving more power to the ken throw and I managed to do a few, but I struggled a lot.
Then I noticed that people on videos did catch the string very close to the ken, I tried the 80-20 and voila! it worked flawlessly and now I can do it "almost" every time. I'm with Dimensions here, it's a feel trick.
I tried, and I can also do a half swirl with the 80-20 position. Haven't tried doubles yet, I guess that will be tricky.