According to Jorge Cervantes and his grow bible the closer to 12/12 when they just start to have 3 sets of leafs will increase chances of becoming female - something like 15/9. This however might cause for a longer veg period.
Taken from here.We did a lot of experiments with light times a few years back using known sat and sat dom clone lines.
With Vegging ie under hid lights.
20/4 produced the most sturdy growth and the most bulk. Best final yield, taken as 100%.
22/2 Less of both. yield 88%
18/6 Studier than 22/2 but slightly less bulk. yield 87%
24/0 Much lighter in all aspects than 18/6. Yield 79%
16/8 The weediest plants. yield 67%
Plants vegged to final pots under fluorescents at 20w per sq ft on 18/6 yield 49%
Have not tried 36 hrs dark but did try 48 hrs from 18/6 veg. The final yield was down between 15 and 20% by var the pure sats the biggest loss in final weight and caused the odd herm, [sats] it did reduce the flowering time by 5 to 8 days.
For the mum lines we have 20/4 to 12/12 gives the best crop weight and bud quality, really thats all I’m interested in.
24/0 will work, but 20/4 gives a better end yield. Autos will flower regardless of schedule, but I believe they would also benefit from 20/4 in the same way.so 24/0 is only for autoflowering plants? So 20/0 is the best obviously but i dont see how theres any way to do that without using a timer hooked up to the lights. its not like im gonna get up twice a night tp turn lights on and off and im not gonna do my dark cycle during the day...