Nitrogen toxicity?

Nitrogen is mobile, meaning already existing N within the plant can be re allocated to newer growth, problems with N always show up on old growth first. Next is to look at where it begins on the leaf, from what I can see it starts on top and works its way inward. With light crispy brown blotches. Potassium deficiency shows itself when leaf edges turn bright yellow, but the inner leaf stays green. Older leaves show symptoms first, and leaf edges soon turn brown. I see you just started flower so it's more plausible. Potassium is mobile too. Yellowing is generally caused by disruption to chlorophyll supply for one of many reasons. It may be a small potassium deficiency creeping in due to a slightly low pH. It could be possibly even be a very small section of the rootzones over watered cutting off oxygen, causing small uptake problem. Foliar application of water soluble potassium to immediately remedy until pH levels can be verified.

It does not appear to be Nitrogen Toxicity. When that happens your leaf will curl inward on itself and the tips are burnt and crispy. It does remind me of a lockout with Cal/Mg however. I think your best bet would be to give the medium a good flush to remove any excess salt buildup in the medium. Typically Mg problems are on the tips of the leaf and they will turn brown. Either way I would give a good flush. Hope this helps.

Nitrogen toxicity is usually displayed by a dark green leaf appearance and down turned leaf tips. If it was severe nitrogen toxicity your leaves would cup and claw. You could literally watch that happen before your eyes when applying food that hot. Have a look at some Potassium deficiencies in your research and see if that sheds some light. Potassium typically manifests on leaf edges and tips and is usually a rust coloured necrosis. Yours doesn't quite line up with that from what i saw, but deficiencies can't display in typical, and less typical ways. Excessive calcium can lock out potassium, and vice versa, so if you are feeding heavy with calmag, that could be a clue. Good luck!