I am assuming the volume of the tooth can't increase? If there is only 20 % of the tooth left I am assuming all that can improve is the quality of it. ?
Well, conventional knowledge will probably say the volume can't increase, only the quality. But I think otherwise, and think my front teeth are building back up a little (my dentist made my mouthguard a little loose there because I was stressed about the pressure on cracks I have in those teeth, and it fits a little bit more snugly now). I've never actually discussed increase in volume with the dentist because I assumed that's what we're both talking about when we say re-mineralisation.
I believe in healing, and I know that we don't necessarily hear about it when it happens. I've posted before on the forum about how I injured my hand and was left unable to move my thumb, and was told by three different doctor/consultants that I'd never regain the use of it. When I did (because I believe in healing and didn't give up, and did tons of stuff to help it heal) I thought they'd want to know all about it and write papers. That was naive. They couldn't get me out the room quickly enough, presumably so they could go back to what they "knew" without having it contradicted.
Having had that experience, and having worked in a medical research organisation and learning about what makes it into published research findings and what doesn't, when people say research shows that something isn't possible I don't spend too much time worrying about that. I imagine all the people, like me with my thumb, for whom it was possible but we're just not told about it. (This is also why I don't pay attention to the idea that PTSD can only ever be managed, not healed.)
If you're talking about only 20% of the tooth left, then obviously that would be a lot of healing. This might not be something you like the sound of, I don't know, but I would be doing a lot of visualisation for something like that, as well as doing the other things. I wouldn't expect the whole tooth to grow back if it was me, not because I think it's impossible but because healing is hard work and that would take the kind of energy I don't think I could give it. Healing from PTSD uses up quite a lot. :)
I was going to say that as long as the tooth still has a nerve in it then it's alive and always working to heal if it can (ie depending on the conditions). But actually the thing that got to me most about my thumb was everyone telling me it was dead - so if my dead thumb could heal...