The rules require that there is a rack line that is 280mm long with a head ball marker and a spot for the eight-ball.
They do not specify how big the spot should be or the position of the rack line in relation to the eight-ball spot which is at the intersection of the two diagonal lines between the corner pockets and the middle pockets.
It is suggested (only by the author here) that the centre of the 280mm rack line should be on the eight-ball spot, because then the rack line extends 25mm (1 inch) in front of the head ball and 25mm (1 inch) behind the centre back ball, so it can clearly be seen if the rack is ’tilted’ to one side (i.e. it is not inline with the rack line) which may give an unfair advantage to the breaker.
Additionally, by making the head ball marker line maybe 50mm – 75mm (2 – 3 inches) long so that it protrudes beyond the sides of the head ball, both players can clearly see if the rack is set high or low on the table, which can also give an unfair advantage to the breaker.
The main purpose of the rack line and it’s markings is to best ensure that all rack setups are equal, regardless of who is setting them up.