Here's a fly in the ointment.
The seeds I've been growing are obviously from a female/female pollination, the really high female to male ratio bears this out. I had yet to get a male in more than 40 seeds started to date.
I went 12/12 2 days ago, and so far, I have weeded out 5 males. These are true males, and so far are not showing any signs of female parts. I'll keep watching them, and post whether or not they are hermies, but right now, it looks like 5 true males from a F/F cross.
Go Figger!!!
Still and all, not a bad ratio. But to answer the question, not even the seed company will guarantee 100% females. They are "Feminized", not Female. You're paying for the better CHANCE at more females.