There are several things that you can do to help get more females, I would think one of the best authorities would be a seed company. Somebody who knows seeds, this is what dutch passion has to say about sex.
From literature and our own findings it appears that the growth of a male or female plant from seed, except for the predisposition in the gender chromosomes, also depends on various environmental factors.
The environmental factors that influence gender are:
* a higher nitrogen concentration will give more females.
* a higher potassium concentration will give more males.
* a higher humidity will give more females.
* a lower temperature will give more females.
* more blue light will give more females.
* Fewer hours of light will give more females.
It is important to start these changes at the three-pairs-of-leaves stage and continue for two or three weeks, before reverting to standard conditions.
PS, there is someone I go to for advice, that swears by this method. They claim a 90% ratio of females.
Peace