This is my thoughts on flushing. You are stripping the nutrients from the roots and medium. So ph is useless at this time. You are forcing the plant instead to use stored nutrients. By forcing it to use its stored nutrients, you want this to happen fairly quickly, so be aggresive.
I do an ebb n flow. I do use a flushing nutrient for 2 reasons. 1. No matter what anyone says it's quicker. 2. They have added sugars in the flushing agent which is a nice bonus. Especially to those who do a long flush. Its like saying, na I dont want these buds to get any better.
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I do an aggresive 3 day flush. I fill my reservoir with tap and flushing agent. ppm 220-ish. Start pump, let run about 20 min, then top feed each one for about 5-10 minutes in 20-30 second intervals going back between plants. I usually get my ppm to about 850-950, then it stops rising. Drain reservoir, fill with tap/flush agent. Switch to 5 feeding a day. after 2 1/2 days of that, I cut water off and turn lights off for a day or so. Super quick flush, super quick evenly dry time due to cutting off water last day or two, and super smooth good tasting buds due to forcing plants to use all of its stored nutrients.