It can be a pain in the ass, but when your committed to regs like I am, you can germinate, grow the seedlings out for a few weeks under 18/6, and then hitum with 12/12. A key point to this, is after 10 days or so under 12/12, look at them very closely with a loop. When you see stamens forming on the males (you MUST be sure) wackem. When the females show, getum back in 18/6 as soon as possible !!! I would never roll the dice and grown a reg out to determine sex ! I do grow some fems too.  Happy growin and Happy 4th