Max 5X. I have plenty of headroom coding using Opus all day so I figured I may as well use Fable a bit. I have found Fable to do a much better job of understanding a codebase and avoiding subtle bugs.
Oh man, I haven't used fable that much, so I can't say much, but based on your experience, I think it seems pretty good. Also, I am on the pro plan, so I can't afford to use fable xD.
Opus has been great for me on building simple features or adding new things. Fable has been significantly better for things like "there are subtle bugs in this complex interface I can't figure out, find them". Opus gets a rough idea for some. Fable actually finds all the tiny subtle issues.