An issue about view analysis

Hi
I saw “View to the Outdoors Through a Facade” file and in its description, it was said: "a quality view is defined as an area with a view factor angle of 20 degrees or greater (or 2% of this full 360 field of view). Actually, I’ve read that research paper about different views to the outdoor, but I can’t understand why we should use 2% to cover 20 degrees of 360 field of view? Isn’t correct to use 5.5%?

Thanks in advance

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