I've put my 6 seedlings under a 300W COB CREE UFO light, atm is a 30cm distance from them, I do hope is not too close. I've only a 140cm height and more would be problematic to manage in the long period.
Hi!
If the led pulls 300W from the wall then I think its too close for the seedlings cos they dont need that strong light. You can raise the light to maybe 70cm distance and if the seedlings start to stretch too much you can lower the light. When they are little bit bigger the distance can be something like 40-50cm and even closer in flowering. If the leds are too close you might burn the plants with heat or excessive lights.
If you don't have a lux meter, you can try downloading an app that performs this function to your smartphone. it is not 100% reliable but you can still get an idea if you have a good smartphone. in the first days your seedling needs at least 4000/5000 lux to grow so I advise you to approach the lamp and try with your smartphone to help you adjust the distance.