That rule of thumb for how many hens you need per rooster is something used in hatcheries where they use the pen breeding method. That's where they might have 20 roosters in a pen with 200 hens. They've found that is a good starting point for fertile eggs, fertile eggs are of utmost importance in a hatchery. But that depends on the chickens, some hatchery flocks use a ratio of 1 rooster to 15 hens. There is really no magic attached to it. They check the fertility of the eggs and adjust as required.
But you are not in a pen breeding situation. You get different results with different circumstances. Some males may have trouble keeping a half dozen hens fertile, these are usually pretty old roosters well past their prime. Dad kept a free ranging flock with one rooster and 25 to 30 hens. Even with a 3 or 4 year old rooster practically all the eggs were fertile.
There is no magic in these magic numbers. Those are usually a good starting point in certain situations. I usually keep one rooster with 6 to 8 hens. No hens are over-mated, stressed, or harmed. Practically all eggs are fertile. One member typically keeps one rooster with only three hens. He typically doesn't have any problems either. One secret is that these are mature hens and mature roosters. The eggs should still be fertile with immature pullets and immature cockerels after a certain point, but with hormones running wild until they mature enough to calm down, things can get pretty wild. You may see that before too long. But as far as the eggs being fertile with one boy and six girls, they will be fertile once they mature enough.