You dont have to fill the screen first vegging, it might help in some eyes but it doesnt always work best. The plants are at the bottom of the screen now. When you switch to flower the plants strecth the most in this stage. By waiting until the plant hits the bottom of the screen is about perfect in my situation "maybe not others". My plants are very branchy and within 3-5 days of flower my screen will be full hands down I will post picks and let you guys take a peek. Like I said not every situation is the same but for me this is what works best for my specific plants.