I'm also looking for a general consensus on this but this is what I believe. If you two establish eye contact and she looks down, you can assume she is submissive but you cannot assume she is attracted because like the poster above said, she could be intimidated. It's rather hard to open a HB who is looking down anyways, so it's less viable to approach.
Sorry to necro but In my experience this isn't always true. The best outcome is as you said when a girl looks at you and maintains eye contact. You share a nice moment. Just enjoy looking at her until she looks away as you said. But if she looks, you look back, and she immediately looks down, it could also mean she simply isn't interested. Girls aren't perfect, they don't always look at someone because they are interested or potentially being opened. Maybe she saw sommething in you as well and decided it wasn't worth it lol.
Anyway I think its important to mention this because we need to respect women's right and not think they are always interest in being hit on