The Slaanesh Nova is 12" and just 2D6 - the unit's LD.
I'm inclined toward the highest Leadership available; it's the way LD-related things pretty much always act (yes, there is a Warlord Trait that switches the tables, but whatever). If you want, rationalize it like the fact that you use the best possible Deny The Witch! save you have available--in a similar way to a psyker protecting the unit as a whole, the boldest and bravest members of the unit can rally it against the psychic onslaught that they're suffering. Yes, it's shoddy logic, but I'm not trying to conclusively prove anything. Indeed, the game as as a whole simply does not have a RAW answer.
I'm just saying that in every FLGS I've played at and among friends, we've always, without question, defaulted to "highest LD available."
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!