It’s called a T marker for a reason, the centre line should not go beyond the head ball spot, the 280mm line protrudes out of the bottom of the rack so that the line is directly through the centre ball of the back row.
The distance between the head ball spot and the eight ball spot should be around 89mm.
The T marker head ball line should be around 76mm wide (1 and half balls).
This was implemented in the supreme rules.
In fact the T marker is not in the rules anymore, never been in International Rules, just a 280mm rack line with two dots on for the head ball and eight ball.
Credit to Andrew Sheldon.