Space requirements and Perches

I have read somewhere that each hens requires about 8inch of the perch.
Keep all perches at the same height and about 24inch apart to stop pecking.

I am new to keeping hens, but have bought and read so many books i can't remember where i read it.
I know what you are saying but i think some of these sellers of housing over do it in the adverts how many they are suitable for.
In know that hens will huddle up together for warmth and safety, but i'd rather know that they are not overcrowded in the house which can start feather pecking and bullying.
I am not going to put anymore in mine of standard size, but maybe introduce a couple of Bantams.