Anything less then a foot and you lose the specularity that makes a ringflash do it’s magic. The one method he didnt show is using kicker lights behind a model when lighting straight on in front- the kickers really make the person stand out in back as well as in front. Btw, you can see what happens when you move the ring away from the model as shows in the overhead 45 deg shot- her face is warmer and darker than her chest which is brighter. That is not a well exposed shot and as a retoucher and photographer, i’d have to spend a bit of time evening out that warm reddish color cast to match it with her chest.

Personally, there isn’t a thing in that video i’d ever teach anyone to use if they had a ring flash.