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12" - 18" depending on setup. Start out on the far end and work in.
Put your hand over the top cola directly under the light, back of hand towards the light. If the light is hot on your hand it's too hot for the plant. You can also keep a thermometer at cola height directly under the light but I think this is overkill, the hot hand test is fine.
If you have heat concerns don't let your buckets go bone dry, the leaves could wilt and die.
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