Cool, the dot thing sure makes things easy. It isn't 100% accurate, but darn, once you get a hang of looking for them, they're SO obvious.
Although, it's also possible it's a hermaphrodite. They're more common in bettas than you would expect. A lot of the breeders (especially big farm ones) use testosterone to increase color vibrancy. If a female makes it in the group, the testosterone exposure at a young age can make them one very confused fish! I saw one recently, it had a smallish white dot, but wasn't completely female or male. I'd say it's about 50/50 for it to be a female or a hermaphrodite.