You want it low (when i say low I mean like medium or below, medium is best) because if the camp spawns in front of your base its possible a thrall or two gets pulled into the camp and surrounded by waves that spawn. This is about the only way a thrall can die using this strategy. So make sure you respond to this immediately and if you need to, pull em out of the fight for the rest of the purge. At 15 distance they dont tend to stray too far from the platform (most of the fighting takes place on it)
Just dont forget to change both attack and chase distance