How Do You Calculate Active Usage?

How do you calculate active usage when people flake off? I think it's useful to look at the metrick, because otherwise you're just flying blind. It's really about fighting that hidden need, figuring out what people want and im building that solution. But optimizing for learning is sort of the key here,. where you say, what's the biggest unknown that i have, and how do i go about answering that question? E'll get two more questions.