Rule of thumb for planning on dropping into your 12/12 cycle is to flip your lights when 70%-80% of your screen is full. I do believe it is the first 21 days(??) of flowering that your plant will continue to stretch, which in this time you still train your branches underneath the screen. Always train under the screen and then let the bud sites grow through the opening until they have stretched enough to be moved to another hole if possible. I'm not sure however how training above and below the screen would effect the plant. But I have always read to train under the screen. Good luck with your grow. I am also doing a scrog this time around. What strain are you growing and how many plants?

MEME