I’m using the Ts600 and it is a powerful light for seedling at 18in she will start to turn yellow . If you look at my seedling photo they are more than 24in away from light now ant seems to like it . In growing them in a 2x2x4 tent my ts600 is at the top of tent on cross bar . My seedling are on the ground .they are stretching right now but that’s ok for me right now I should be switching rooms in a week or so . 24 - 30 in away and you should be good . Hope that helps