i have heard incubator temps can determine sex, can't remember specifics, don't know if its true- other theories are the hen determines it, may be something to that, i quit hatching for that reason too, one thing a friend does is caponize her extras and let them freerange to take care of bugs
My understanding is that pullets can handle cooler incubator temps better than the males. Males can handle higher. So in theory, if your run cooler the males die, therefor the ratio is higher for pullets. Then just do it the opposite for boys. Not sure how much truth there is to that otherwise, hatcheries would really use that.