What does that video illustrate about an Air2? You do realize an Air2 would also be on a short hose, right? Same length as a necklaced octo. Where you see bad equipment choice based on old myths, I see bad divers with poor training. The funny thing is it was a traditional octo that started free flowing at one point.
Anyway,
@Chris Richardson, over the shoulder is fine. 18-22 inches on a necklace is how you would set it up with your primary on a 40" hose under your arm. This is called the Streamlined Open Water setup and is a fantastic, clean and tidy setup. Alternatively an Air2 also works especially well with this setup.
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A 5 ft. primary hose is a really bad hose length, IME. It's unwieldy as it's too short to secure under most average size guy's bodies. A 7 ft. hose doesn't make much sense in open water as its purpose is for single file cave exiting. 40" under the arms works great. Short enough to be streamlined, yet long enough to do a comfortable air share.