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This is the Floura analisis of my sheet:
As reference, u will need from 150 to 300 PUR in veg state per m2. Average 200-250. With 3 18w tubes, u get 9,431 *3=28,29PUR.
28,29/200=0,14m2 (1 1/2 sq ft) is the grow space allowed for that light (using decent reflectors).