How accurate is nub theory?

So back with my kids (ages 18,16, and 12) nub theory just wasn't a thing. In fact ultrasounds early on just werent a thing unless there was some medical issue. So when my tech asked us if we wanted to know and explained nub theory (which I had looked up since it has been talked about here a lot) I of course said sure! Well, she said 80% sure its a BOY! We have all girls so this is a great surprise, but I want to know if its really accurate. US was yesterday and I was 12w6d.