From the maps it clear. Dagoretti...Limuru...tigoni...you go towards kiambu/Ridge ways..Ruiru..Thika...all the way to Makuyu. I'd say 10Km strip of land. It also seem like most of land the mzungu took was around border. It seem like a border land btw kikuyus, Kambas and Maasai.
Generally the white highlands consist of
1) Land mzungu felt was vacant..this was mostly Maasai nomadic land..which may seem vacant..most of Nakuru,Nyandarua,Uasin Gishu, Tranzoia and Laikipia...was basically Maasai land.
2) Border buffer to prevent tribal clashes...I think Central,Ukambani, Borabu-Kipsigis, Sugar-Cane in Nyanza, part of Nandi can be classified thus...generally the Brits carved out 10Km buffer around hostile tribes and gave out the land to their settlers. This really helped prevent perennially savages from attacking each other and is commendable.
3) Forested Land...where few okieks and likes lived...Kericho, Kiambu {limuru, tigoni}..mzungu took over and deforested the places using Africa labour.
must have been due to population ..for Central there was alimit to which you could push people ...
one gets a feel of white highlands by branching off thika road.... you have to trave about 10 kms before you get to the lands left for mwafrika peasants settlements.... take any road leading to Wangige, Banana, Mangu, Gatanga, Kandara........ as ypou go farther away from Nairobi the large farms are still intact but closer Nairobi they have been turned into real estate