In my studio 99% of all portraits are done at a distance of 1.5 to 2 metres. Why?

In "Western" style societies this is the distance that two strangers set when they are engaged, interested, attentive, respectful, but not invasive of personal space. This gap is so familiar and consistent that facial features, ratio of nose to ears, chin to neck, etc, just look "right".

Once the distance is known framing is organised by choosing the appropriate focal length; long focus for tight face portraits, wide angle for half-figure, and so on.