For me, I couldn't tell you at what distance I would feel at danger, as a general statement. It would depend upon the totality of the circumstances; perpetrator(s), are they armed, with what, overall demeanor, am I by myself, is my family present (I value their lives more than I do mine), what condition am I in (injured, winded, etc.), etc.
The definition is kept "fluid" for a reason, and I think it's a good one. It limits Monday morning quarterbacking because other folks can't put themselves in the exact situation you were in, because they weren't there.
