Im just starting a grow with two White Widow plants in a tent. First off my light is a DIY COB Cree 300 watt kit from Timber with 4 CXB 3590 lights. Not pushing a source just the type of light. Also Im in a small 2'X2.5' tent. 7' tall. At 30" from light to plant these will push 900+ PAR readings easy. This grow I started the seeds under CFL for about a week while using my tent to dry previous harvest. 1 week after sprout I transplanted into the big pots in the tent. I also invested in an Apogee PAR meter along the way. I calculated the DLI and PAR for a 18/6 and a 12/12 time period and made a little sheet and attached it to the light. After doing a lot of reading here there and everywhere I could find including major university research departments that publish and have websites. The university ag programs all deal with other plants but I try to follow the tomato data in general. After all that the best info/guess I could come up with is on 18/6 seedlings prefer a PAR of around 200 and veg about 325 PAR for a DLI of about 21 after a couple nodes. I had 200+ for a week. The day before yesterday I turned it up to a PAR of about 350 PAR for a DLI of 22.5. Light was about 30 inches above leaves. Last night I got home and checked a couple hours after the light came on and one of the plants had a growth tip with a few small leaves turned upside down. I thought it was caught on something but no it was intentionally doing that. Plus one of the second node fan leaves was turned 90 degrees away from the light. Didn't get a pic sorry. So I backed off the light to a PAR of 325-330 for a DLI of 21ish and raised the light up to 40". PAR reading taken after raising the light of course. Temps have been in mid 70's for the highs. Im adding humidity to the room to keep it in the 50% range but no higher than 55. Low about 40ish. So this morning when get up and check the babies and the growth tip and its leaves were back facing up and the fan leaf was also facing up. The other plant looks as good or better. All I can take away from this is I was using a bit too much light. Its surprising how a small tweak of lowering the PAR made a difference. Its somewhat frustrating to not have any real hard scientific info on what light, PAR and DLI cannabis does best with and I am surprised that I cant find any real hard data reported by the professional growers about this topic and how they calculate their lighting and whats working best for them. And no I don't put much faith in what people trying to sell lights have to say because it usually comes back to "if you buy our light,,,". I mean how many times have I seen in print be it stoner science or elsewhere that on one hand you cant have too much light and on the other too much light is bad. Im still trying to find the holy grail of lighting level by those who do it for a living but cant find it.
Having too much light and having your light too close are two different things. I don't see a 300 watt light being too much at all. You also have to get plants used to certain light levels. Different strains have vastly different tolerances to light. I had a sativa strain I could not get too close to the light. In the same garden an indica leaning plant needs twice the light distance from the same light. Just because a plant exhibits light stress doesn't mean it's too much light. The light generates surface temperatures on the leaves that are much higher than the air temp in the garden. Some plants have low tolerance for that. Have you put your par meter in sunlight? What is the reading? Many people don't realize that high power LED's require more light distance typically than HPS even. I run mine at 24-30" after experimenting with trying to run them closer without good results. The sativa could tolerate 14" without singing off pistils or burning leaves.
Going by what the vendor of the kit recomended I could have even gone with the 200 for my space but oh well. The Apogee meter has a setting for sunlight or electric lighting and I never bothered trying it outdoors. I came already calibrated and their website lists how to check it and some offset numbers to factor into using it with LED. It's not a lot but I try to factor it in when using it. I have spent a lot of time using google trying to find info on what medium to large scale growers use for their light level targets. Unless i missed some there just isn't much at all. Maybe they consider it proprietary info for their success. My other grow I did use a thermal gun to check surface leaf temps but not on this one so far. Like nutes or amendments in soil I'm realizing with lights less may be more.
Sounds to me like everything you are doing is fine. Do not worry too much about the light from your 300w, lets be real anything over 40cm-60cm+ from the canopy will be safe in early veg after transplanting seedlings. You are overthinking the whole PAR level measurements.
I don't see this as overthinking it. Some growers that have been at it for a while have a feel for what works for them in different aspects of their grow. Newer growers like me have not gotten to know what to expect from our grow setups yet. Be it soil, environment, lights, care etc. Plus I like documented facts. So much of the advice here and other forums is spot on and jives with actual research from what I can tell and there is a whole other end of the spectrum of stoner science. I'm not into answers that people put up as fact without some kind of evidence instead of the " it's what everyone says" thing.
Well you dont have to have facts or evidence to always support an answer, sometimes its just simple logic. i highly doubt there are many people out there that purely adjust their light distance to the canopy through PAR measurements. Yes seedlings are vulnerable to light burn but it REALLY is not difficult to set up lights properly, even for a first time grower. PS i dont mean to argue with you im just being honest