The right next step makes you uneasy to think about and still looks possible. If thinking about it turns your stomach it is too big, and if it bores you it is too small.

What the research says

A careful review of the experimental evidence argues that exposure works by building a new memory which competes with the old one, that the feared thing did not occur, rather than by wearing the fear away. The practical consequence is that a step should be chosen to violate what you expect, not to keep your anxiety low. This is a theoretical account drawn from laboratory work, so it is the best current explanation rather than settled fact.

Strength of evidence: one line of research. A single study, or a theoretical account.