Sounds right to me. You only take the hit for the powers used. Also, it doesn't matter how many times you use the powers in that scene. However, if a scene is long enough, the GM may be justified in calling for an extra roll in the middle. For example, if you had a negotiation that turned into a fight that then turned into a car chase, you might consider that "one scene" but it might be long enough to be worth having an extra feeding check or two.