I actually see a few issues going on. First and foremost, digital meters do not work with organic nutrients. Digital meters are reading levels of metals in the solution. Organic nutrients, while they do contain metals, they also contain a significant amount of carbon and other particles with a neutral charge, and therefore will not be read accurately by the meters. Now this brings up the good question of how do you check pH in soil? Well that's a good question, and my answer really has been that I don't bother checking when in soil and using organic nutes. Now this is not to say I have not had issues, but I don't fret about them as my results have still always come out where I would like.
On the topic of nutrients, 12 mL per gallon just seems like WAY too much food in a single dose. I see that
Bio Bizz actually recommend about 15 mL per gallon at the heavy feeding stages, but I personally would not go that high, unless my plants are totally blowing up. So I don't know if that is any part of your issues, but I always like the idiom "less is more". And if you are trying to flush out excess salts you are actually trying to bring the pH up. Remember 7 is a neutral pH and Cannabis needs a slightly acidic environment. So if you are removing the nutrients that are keeping the pH below 7 the pH will rise, regardless of what the pH of the water you are flushing with is. If the pH drops below the ideal zone, then a flush is what is needed, but if the pH is too high it is likely a lack of nutrient retention within the soil.
Since we are talking about soil, Light Warrior is designed for seedlings and fresh cuttings, it was not really designed for mature plants. It was not meant to hold nutrients the same way regular potting soils would. I would highly recommend a change of soil, in the future, for your mature plants. If you are set on
Fox Farm soils, and they are very good soils, I would recommend Ocean Forest. It does already have perlite in it, but more is not going to hurt. In fact I tend to add perlite to any bagged soil I buy. More aeration is not going to hurt my plants, so I am not shy with the perlite. I usually like to have almost a 1/3 of my soils to be perlite, the results for me have been great root growth from my plants. Ocean forest also has a good amount of food for plants, so in the veg and early bloom phase you can use less bottled nutes.