Here is the chart I found

Looks like 18 inches from the top of the plants. Seems too close to me though
Johnny is the HVAC guy, but I think this is similar to what he is recommending
https://www.amazon.com/Lasko-2554-42-Inch-Curve-Remote/dp/B000RL1WNQ/ref=sr_1_2?tag=greenrel-20