I added a second DIY light and decided to move them down. I left them at 15cm from the plants. One of the plants started showing symptoms of heat stress, so decided to replace the DIY lamp with another Nielle 140w (600w supposed equivalent led watts, you know...). I moved the lights to 40cm from the top of each plant. Plants are recovering fine, the heat stress is less visible now, but still present in one of the plants, still recovering.