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You might measure 18 inches but those last few inches or so won't get direct light, some of your tops will be in shadow and therefor not optimal for bud development. Try not to grow to the ceiling.You are probably right. Maybe 10 inches for the first and 20 for the 2nd? If do that , it should allow for 18 inches from the 2nd net and the light. The plants are around 6-7 inches tall now.