Showing signs of nutrient burn, where you feeding by ppms or suggested serving on bottle at 1/4, half amounts? From that pic, looks like another couple to few weeks. Stop feeding giving plain water is just as good as flushing. Flushing compacts medium, chokes roots, flushes oxygen out of medium smothering roots. So close to harvest none of that probably matters just something I and countless other growers prefer not to do. Feeding by ppms gives plants what they need no more no less. Getting run off readings on fresh water days will give you an idea of how much or little the food you gave they ate. Once a understanding of plants appearance is achieved the runoff readings are irrelevant you can feed ppms by plants size, growth rate, and appearance. Personally when I used this method I would only give ppms needed to get me back to a good ppm level. Like if I gave 600 ppms fresh water reading was 550ppms plant ate 50ppms over 2 days by the next watering I'd assume they ate 100ppms if I'm watering every other day. I'd only give that 100ppms not another 600ppm every feeding. Getting the run off reading will allow you to see if nutrients are stacking up allowing you to only give plain water until ppm levels drop