Man, i feel your pain like no tomorrow. i've legit done all i can to chase that dang number myself. tried everything from new probes, different brands, different brands of calibration fluid, micro bubble, fresh air line from outside going into the DUAL co2 scrubber, tablespoon of RO water in the co2 canisters, different brands of media... everything. still 7.75 lights off 8.0 at peak... BUT ITS STEADY, CONSISTENT... so i've let go of it and let the tank do its thing. all of my other parameters are on point, SPS dom tank and the colors are full and vibrant, and my fish are happy, healthy, and beautiful. so as long as you bring it within "range", even if its at the low end. ride it out. let your tank stabilize and tell you where its happy.
That is the sump and dosing system of a full on OCD control freak, and I love it. I applaud your organizational skilz. Well done sir!