Darken the floor. Even if its just some cloth. With the subject at a distance you would never be able to totally trust the cam settings cause that white is not gonna allow true sight to judge the shoot.
The white floor is making light where there should not be. A french box wouldn't even help much.
I would get as close as i could to an edge with the tripod.
I would put a black felt cloth over the white roof top. Maybe, 10ft x 6-8ft. if the retainer wall is white also put black felt there also. then you will have the ability to make better adjustments.
I had the same thing happen in Manilla but not a roof.
"""
What you do with what you have, is more important than what you could do, with what you don't have."
> > > Knowledge + Action = Wisdom - J. Corbett 1992 """"