[Negitivefrags]

That is not an example of JK Rowling judging her own abilities. Her abilities were judged by the market when she published her books.

It's possible to know one is good at something, it's just not possible to judge your own abilities.

[pg]

I'm not talking about an existing book. I'm talking about the case where she's trying to predict the chance she'll be able to write a new book, before she's started it.

Her previous books should give her confidence that she'll be able to write another. That is one way in which she can judge her own abilities.