I have done the scrog/screen a few times. I had the same question that you did and got little help. I learned to put the screen on the day ya turn to 12x12 or 1 week after. The whole purpose of the screen is to hold the budding branches up when budding other wise they just fall over from the weight of them. I know this old but strains make a difference as well. Y a will have to experiment. Make sure you are all feamales before putting the screen on them.