You've communicated enough when your audience begins to communicate, on your behalf, with itself.
This morning Scout was grumbling about how hard it is to focus on the ISAT tests. I realized I was hearing preemptive excuses.
And so—still in bed—I began to intone sternly (think Gregory Peck, in To Kill A Mockingbird), "Scout …."
But I actually wasn't sure what I was going to say next.
"I know, Dad," she said, "I need to bear down."