How many people, do you think, go to a doctor and say "I think I'm mentally ill"?

Ah, when you phrase it like that I realize that my estimate is rather low. Near vs. Far mode, I guess. Since it's relatively unlikely that someone would do that if they weren't actually mentally ill, and some mental illness is mild enough that one wouldn't bother, and a lot of the severe ones could prevent someone from consulting a doctor on their own, a pretty low proportion of the population seems reasonable.

Does that line up with your reasoning?

edit: I think that part of what was muddling me was that your original phrasing ("talked to a mental health professional about an assessment or a diagnosis") was sort of unclear, so I resorted to nearby heuristics rather than trying to parse it properly. We might want to fix that up before putting it on the survey.