http://www.autosport.com/news/report.php/id/101147
From the Autosport article:
Whiting said that "any driver defending his position on a straight and before any braking area may use the full width of the track during his first move provided no significant portion of the car attempting to pass is alongside his."
To further clarify the situation he later added: "For the avoidance of doubt, if any part of the front wing of the car attempting to pass is alongside the rear wheel of the car in front this will be deemed to be a 'significant portion'."
So you don't always have to leave a cars width, at least until a part of their car is alongside yours, then you do.
From the Autosport article:
Whiting said that "any driver defending his position on a straight and before any braking area may use the full width of the track during his first move provided no significant portion of the car attempting to pass is alongside his."
To further clarify the situation he later added: "For the avoidance of doubt, if any part of the front wing of the car attempting to pass is alongside the rear wheel of the car in front this will be deemed to be a 'significant portion'."
So you don't always have to leave a cars width, at least until a part of their car is alongside yours, then you do.