Depends on your definition of “sure things.”
If it’s just “will give you a warm reception”, I’d say that sounds like a reasonable number. If it’s “will end up in your bed”, it’s not generally going to be that high unless you’re on a real hot streak.
Whether it’s worth going after girls who aren’t warm to you or not depends on your goals. If you want to really develop your abilities with women as a skill set, I’d recommend it. The women who make you work harder for it train you more. Depending on your disposition, you may also enjoy these lays a lot – for my money, the most enjoyable lays are a.) the girls who really, really like you, and b.) the girls who don’t like you at all. One’s a magical connection, while the other’s a triumph of skill and conquest. Most guys will feel the same about a.), but many won’t about b.), and that’s fine.
If your goal is just “hook up with girls who really like me” and “do as little work as possible to get my lays”, you can just look for girls who are really warm to you, and be fine with it.