They look fine to me - but perhaps that's just the photos... A magnesium deficiency will affect the lower leaves first turning the tips yellow which then moves towards the stem... as the yellowing moves towards the stem, the tips become brown and brittle. If this is what you're seeing, the first place I would look would be to your PH readings... these should be in a range between 6.0 and 6.5 (in flower, I like to get to 6.2 as more micronutrients are absorbed at that level). Your diary doesn't indicate a PH for this grow which makes me think you don't have a PH pen... this is an INVALUABLE tool for anyone growing cannabis and I would urge you to invest in one.... they can help you avoid a myriad of problems...
Good luck!