The numbers ("95%", "50%" in your example) are good to illustrate the differences. But sometimes hard to obtain.

Indeed, which is why I made them up! :P

Problem is I can't even provide a good estimate for myself.

I can think of one time when this happened to me and was annoying, about 2 months ago. Every other time that I can remember when someone has contradicted one of my self-assessments, I've responded positively, because I enjoy getting feedback about myself.