I think the overhead doesn't come close to the one millimeter accuracy required for the shot O'Sullivan faced. Also, it requires an accurate "before" image and if the black was always spotted wrong, the "before" would have been wrong as well.the chinese promotors have a digital overhead method in their events. not sure about its mm tolerances, but it's probably better than just eyeing it up.
Some ball markers have sort of a skirt under the ball down to a small semi-circular hole. If you center the spot at the middle of the half-hole the ball will go right on the spot. (I've previously posted a different design that can also spot accurately.)