I think just marginally back of the ball of the foot is right.
I am just moving from SPD to Look Keo type
I saw my existing cleats are giving my left foot a slightly toe in position.
I will avoid this in setting the new ones up. Left knee has been ever so slightly stiff for a while.
It is not just front and back , also side to side and angle as well , all need to be right
Most people like some float -- i.e. the pedal will let you take your natural angle to some degree.
It means the rest of the setup is slightly less critical