The closest I have ever gotten to testing this theory was to take 6 regular seeds, not feminized, and grew them under 12/12 from the day they sprouted through the soil. All 6 were true female, none hermie. So, let's say subjectively if you have a 50/50 chance of getting male or female with a regular seed, the odds of planting 6 and getting all females would be 1 out of 2 to the 6th power, or 1 out 64. That left me assuming, unless I got pretty lucky, that the 12/12 from seed theory may hold some credence. Try it out if you have any room in your flowering space. A plant grown 12/12 from seed won't take up much space. I bet you will get more females than males doing this.
Note: I do not know if there is any predisposition for cannabis being either sex, so I know it may not be 50/50 male/female.