From the books there are a alot of instances where harry will not use magic to do things, because his control is not great enough.  As far as I see the rules really don't support this.  The discipline roll is only important when you are using more power that you can aesily handle. 

What about a situation where you only need a little power, say enough to lift a chair, but you then want to move the chair through say an obstacle course.  If would seem this type of spell should have a low power requirement, but a high discipline roll needed. as far as I see, there is not current wy to simulate this? Thoughts?