But in NS the Ajah head specifically says that you must defer to anyone stronger than you. So technically one level is enough, though it's rarely enforced. I also don't think that Moiraine and Siuan are good examples for this custom as they had only been exposed to it for minutes! It's probably better to refer to the main series?
What was the strength difference between those two black Ajah who argued about deference in front of Elayne after they were captured?
Fan of Lanfear