What is the appropriate recording volume in the camera?

About minus 15.

You can generally hear clearly if you speak softly in the camera recording. That is, if a person is next to you, you can still hear you speak in a low voice. The camera can hear it. If a person is next to you, your voice can be heard clearly. If it's so small that the people next to you can't hear it, then the camera will definitely not be able to hear it, just like human ears can almost hear the distribution and should be slightly more sensitive than human ears.