I thought it was 0, I'm sure thats what leaving sphere decisions say..if you are a secondary power. But your former master always instantly declares war though..not that I have ever played a country with heeps of military score to scare the sphere leader.
you need to have negative relations (or maybe 0) to leave sphere. The leader get a CB and will probably attack. I would wait until they are busy in a great war or such and cant deal with you, and build up your military to scare them.