I honestly dont know the distance as my setup is not ideal. The light is hanging sideways not overhead, It still works completely fine, I honestly dont see a problem with the light im using, my plant looks fucking awesome in my opinion and this is my first time.
Soil is generic store bought soil I honestly cant help you with that question, I didnt do enough research on getting ideal soil and the soil i used has no info on the internet about it at all. I know it has fertilizer in it, the amounts I have no idea and how long they release for I dont know. I water daily, they are in 8.5 L pots, I let them dry out quite alot before re-watering. I do not use nutrients at all just plain water, so I have no idea how they'd be getting nute burn at this stage. My plants do look relatively close to being ready I thought, so maybe the plants are using what resource they have left and wilting?
Also possibility is because ive been using water for so long, the ferts in the soil have been flushed and its not getting enough nitrogen. I think I will add a small amount of nutes and note what happens over the week. I think you're right about it being N