“How do I know how much compression to use?” Just use your ears. That’s a correct answer. It’s a well-intentioned answer, but to someone with limited experience, it can be a frustrating answer, because what happens when your ears don’t tell you anything?
It’s a little like telling someone who’s uptight to just relax. So let me try and answer this with the teacher perspective. Let’s refocus the question.
Why aren’t your ears telling you anything? In most cases, you are going to choose a tool because there is a problem that you need to solve. You’ve identified that problem, and you’ve identified the appropriate tool to solve it, which means that how it works.
When you apply that tool, the answer can be somewhat binary. Did you solve the problem or did you not solve the problem? There’s a saying, “When all you have is a hammer, “everything looks like a nail.” I think oftentimes we grab a tool that we want to use and we start stamping it on everything waiting to make it sound better.
Sometimes it will, and sometimes it won’t. But if you don’t know what you’re looking to change, you’re going to have a hard time understanding what it’s doing. I believe the answer to your question has more to do with intent than anything else.