Don’t waste your money on one of those probe style ph meters especially in coco.. The best way to get your root zone ph in coco is to make a slurry of coco from your pot and distilled water and then ph your slurry. You should always feed with ph of 5.8-6.0 going into coco. Watch your Ec/ppm of your run off to make sure you don’t get salt buildup which will cause ph to be off in coco.
Also a good method for seeing what's going on down there but I disagree about continuing feed at 6.0
Great, you did your slurry and have now identified the problem. How are you going to be able to fix it with live growing plants? You can't amend the soil with lime or some other amendment that will level out PH unless you're talking about testing and adjusting pre planting.
This is why I grow in soil. Coco and other synthetic media require much more attention in regards to keeping things within spec. Certainly not impossible to get max flower but why make your life harder unless you just like to be on top of your girls? If I'm a large grow op then coco may make sense since all my parameters can be controlled through room pre planning but in a tent it's much harder to control those parameters. One of the reasons most large grow ops use synthetic media like rockwool is because of ease of disposal along with its water holding properties. Media like hydroton can be reused reducing cost. I use Roots Organic Original for my media because it incorporates soil AND coco along with a healthy dose of mycorrhizae. If I have to be away from my girls I know that soil will be there to take up the slack. Granted it's not the cure all but it certainly makes things easier.
I'm not discounting your method because there are a million ways to skin a cat but, having worked in large commercial grow ops, I see a lot of home growers worrying about things that have very little effect on their grow but because the large grow ops do it they just assume that it applies to their grow. Keep it simple, stick to the basics and just use common sense. You'll do best if you stick to this mind set.
OP, Grim is correct in that probe meters aren't the most accurate but, again, you don't need pinpoint accuracy. You just need to know what the problem is and correct it. As long as you stay within parameters your plants will do fine. I use the cheapy probes and they do just fine. Not all that durable but, again, I'm not a commercial grow that depends on their bottom line to survive. I don't need to squeeze every ounce of performance to maximize yields.
This probe should work just fine.....
https://www.amazon.com/MASiKEN-Ther...9Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=