I'd say, the practise that takes the most guesswork out is to measure the runoff ppm every time you water, and make sure you runoff 15-20% once a week when you water to get a good idea on how the plant is uptaking the macronutrients available, and to give the soil chance to remove the macros that weren't used in the last feeding if you are adding any nutes via liquid...if it is steadily rising, there is a lockout issue due to pH, or the plant has slowed down on feeding. I find my plants slow down their uptake around week 7 or 8 of flower and focus on ripening, but yours doesn't look that far along that is should be cutting down on feed.
By the time deficiency, lockout or toxicity has set in, it's usually too late to effectively remedy the situation. You can catch it fairly early if you monitor your runoff and tailor you feeds by how much the plant is feeding compared to how much general guides run.