Hi-I've grown NL dozens of times, Usually Nirvana Feminised, The yellow spot problem , I still get it no matter what I try. Someone above suggests Magnesium deficiency, but I routinely add a sprinkle of Epsom salts, so I don't think that's the problem. I grow in Bio Bizz soil with Bio Bizz organic nutes, You don't mention what soil you are using, but are using fox farm nutes. I think our common problem is a little bit to do with over fert during the young vegging stage, as the yellow spot thing seems to disappear when the young plant is around half vegged. I think its a common problem associated with soil being a little '' hot '' for a while until the plant is large enough to use up the available nutes. I know its not a very scientific analysis but its the only sense I can make of what would be a common symptom. The good news is, NL is a tough tough strain and can handle it, so carry on. If your grow follow the usual path which I am familiar with you will end up with really good weed, but your plant will look an absolute mess by the time its done. Everything else in your grow seems really well balances , temps humidity etc all A OK. Good luck mate. Reason I mention soil is that from your pics I don't see any pderlite or other drainage enhancer there---hows your Drainage, are your plants getting some form of Oxygen into the root mass??