When to LST your plant is determined by your flower light.
With my 400 HPS, I train and let the branching resulting from that training get 4-6" long then I flower.
If I had a 600 I'd flower when that branching was 6-8" long because of the penetration difference in the light.
With a 250 or fluoros, Id flower at 2-4" because of the penetration difference.
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