The leaves that look like that are scattered throughout the plants. A fan leaf here and there. Those were the most severe but many of the leaves are showing early signs of that same problem. I'm growing in Fox Farm Ocean Forest using Technaflora's Recipe for success. I'm in the fourth week of flower.
I certainly could be wrong but I use the "recipe for success" and IMO they should call it recipe for ph crash, as it has caused ph jump for me and my partner to the point that we were ready to scrap our whole grow. Use those nutes sparingly until yu learn the way they react to yer specific grow, and monitor yer ph REGULARLY. Again, I could be wrong but I would bet yer sitting at about 7.8ph. If I am right, yu should flush yer system with ph ballanced water for about 3 days then add back yer nutes at about 1/4 to 1/2 strength. I would mix up nutes and allow them to sit for several hours then test ph and balance if needed, then add to soil. Yu may want to add a little "grow" for the nitrogen as all yer nutes look like they may be locked out. If yu are foliar feeding, I would stop for now as those leaves look pretty cooked. Yu may want to PM Hamster Lewis as I believe he is a dirt farmer and could verify or recall what I am saying in case I am missing something. I hope this helps. I hate to see a good grow bad. Good grow mojo to ya.
IMO if you dont want to invest a few hundred in a quality ph meter then I would go with the droppers. I have tried four different brand ph prob meters all under $40 and none of them worked. I have been using the dropper ph meter lately and its pretty simple.
I just picked up an Oakton PH2. This tester cost $100 at the hydro shop but i found it online for $60. No little screw driver to calibrate, just push a button and you're good. I love this tester... The results are almost instant and its really easy to use. plus its water proof.