Well, it's a starting point. If experiment showed the advantage to be not so great, then either have a non-standard 30mm or 35mm kill at the same distance, or a 25mm closer to. I was thinking of having the same distance, from the point of view of course setting, wind exposure and so on.
We had a report at the SWEFTA AGM yesterday from someone who has done some back to back testing of positions and he says that his results show a 30% or more reduction in group size using the disabled position.
If you translate that to the disabled position group being 70% of the size of the conventional, then ideally one would have a ( 40 x 0.7 ) = 28mm kill.
The study that RobF mentioned should add useful data.