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I suppose I'd start using all 4 at about 15 weeks old or something, it's a little hard to estimate, it depends on whether everything has gone hitch free or not, at about 6-8" healthy, I suppose. Myabe earlier, you'll have to play that one a bit by ear.
About an inch away from those plants making sure with your fingers the plant doesn't feel hot, you can see where the light reflects on the leaves at that distance, so try to aim for a nice coverage.

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