I think the most important question to answer first is what are you trying to accomplish. I think far too often, TDs are mixing goals and not accomplishing any of them well.
If you run a multi-day event with finals on a dedicated day, I appreciate a B division for the purpose of giving out of town bubble players something to do. In the case, I think the restriction should be be pretty high, really only trying to restrict the people you don't expect to be fighting for a qualifying position. Basically restrict the people fighting for the buy, not the bubble.
If your goal is the give something for people who are not able to fight for the bubble something worth competing for, then I think this is a different threshold. In some ways, I think making people choose to play B/Novice actually works better for this.
If your goal is to draw new people and give them a task of competition, I think a tournament count restriction might be better than rank.