Light height and brightness

I have the same light. About 30" to deliver 400 ppfd. After the second week, you can go down to about 24" to deliver 600 ppfd. 20-22" during transition to flower, then 18" during flower to deliver 900 ppfd. If you're ever unsure, find the light on Amazon, there's a ppfd map on there. Placement of your plants in relation to the center of the matters. Look up the map and you'll see what I mean, they look like a grid with numbers inside. If you use CO2, you can go up higher on the ppfd and put the light a little closer.