How do you know your found your ID/Personal Line?

The most confusing part of this is the shoulder - I get where you start, but I don't get if it curves inward (e.g. Narrow, Double Curve, Petite) or if it falls a bit outside the bust (e.g. for Soft Classic and Soft Natural)

For those who are sure about their Personal Line, how did you know you're drawing it correctly?