Ok... my 2 cents. 
My brahmas are heavy... and although there are lower perches available, they roost on the top perch. My roosts are a ladder type roost, with the top roost being about 6 and a half foot from the floor. They start at the bottom... and one by one make their assent to the top. In the morning, one by one, they come down
I think if you provide a way that they can comfortably go up and down... they will roost where you want. The youngins fly on and off the roost.... the big girls take their time.

My brahmas are heavy... and although there are lower perches available, they roost on the top perch. My roosts are a ladder type roost, with the top roost being about 6 and a half foot from the floor. They start at the bottom... and one by one make their assent to the top. In the morning, one by one, they come down
