Do not approach within arm’s reach (fingertips ourstretched) without them facing you. This is the approximate ‘personal space’ boundary.
I think that this is culturally dependent and not universal.
As soon as they back up even slightly, stop; you’ve gotten too close to them. If you find someone constantly edging away from you, adjust that distance upwards.
I think that this is culturally dependent and not universal.
This seems to me to be a good heuristic.