soil PH runoff

I typically do not get runoff when grown in soil but that's because I use amendments and my soil is dense so if I'm getting runoff I have overwatered and with runoff and amendments it's wasted nutrients leaching the medium and starving the plants. But if your going in at 7 and your runoff is 6, its closer to 6 IMO or lower as the 7 is getting lowered to a 6. Now if you really are using soilless potting mix and not soil or soil with a bunch of perlite, its fine to get runoff. I also see your using liquid nutrients so its always available to the plants and that's not an issues like amendments, I would monitor the EC/PPM of the runoff as well, they may have an effect on your PH swings. High EC = lower PH. Putting enough perlite or extras to help it aerate/drain in soil will also prevent overwatering it. When doing stand alone soil ph tests in the slurry method I've always been instructed to use distilled water. as you want the PH of the medium, and distilled water is neutral, dead middle on 7. so where it goes from there is indicative of where its at. 6 end result is not bad. I'd like to see it aorund 6.2-6.4 but 6 is within the lower ends of normal. The first bit on runoff will always be a bit higher in EC and lower in ph, let it drain a tiny bit before collecting your sample for testing PH/EC. Good Luck!