A plant can only absorb so much light over that it is detrimental. Unless you are using c02. Being newer grower I didn't think you were using c02. There are ppfd phone apps for Iphone or an android phone. I use the 1 for android. My wife has an iphone both phones read damn near the same so I don't think it matters if it's an iphone or and android. It needs different ppfd during different life cycles. Dr. Bruce Bugbee explains ppfd in detail on youtube. And lots of other ppfd explainers on youtube. Just sift through the goofballs on there, as with any social media. Hope that helps. Just wanted to pass on things I have learned about the sf400 since I had the same light. Your seedlings look just fine to me as well.