Area needed to cover in cm2 for Norwood scale

[Dav7]

I'm just wondering if anybody knows of a rough estimate as to how much space needs to be covered in a HT per each stage of the Norwood scale, e.g. let's just say, NW2= 50cm2, NW6= 200cm2 or something like that? Obviously that is incorrect, and obviously each person has different scalp measurements, however what would be the (on average) amount of area needed to be covered for each stage of the Norwood scale? I ask because I want to estimate how many grafts cover an area and at which density overall.