I have tried it various ways.
In the end, seeing how they act with different amounts of space (and because I do not free range them, although they have large runs which are roofed and windbreak-ed for wintertime), most of mine get 15 sq ft per chicken PLUS ample run, and I just refuse to give them less than 10 sq ft apiece even in my most 'crowded' pen.
They could certainly live with less.
But they act different. Less relaxed, less individual.
(Also, remember that up here there are many days during wintertime when (despite roof and plastic and hay and all that) they just do not want to spend much time outdoors in their run. For which, having a good amount of indoor space REALLY helps keep 'em civilized with each other.)
JME,
Pat