How does it decide what percentage "junk" to make each file? I know there is a bar you can set that you can decide between "relaxed" or "strict" but that doesn't quite help.

can one of you explain it??

I am not good at using it myself, but I have it set fairly relaxed. Otherwise it seems to see good files at Junk. As a general rule, any file under 2,000 KB (2 MB) is generally junk. So you can train the filter that way.
If you see a file at 200 or 800 KB & it's a song or video, then time to junk it. Select it then press the trash can icon.

There's also some good tips here How to find music And Beware of Fake files in search results