Okay so if your feed is going in at 6.5 and coming out 7.9 then that means 7.9 is the middle point. Meaning to calculate for your actual pH soil the equation would be
x = (7.9 - 6.5) + 7.9.
"x" is the actual pH of your soil before watering it.
"7.9" is the out
"6.5" is the in and we subtract "7.9"(out) from "6.5"(in) to get the difference. Then depending on whether or not the runoff came out higher than the input or lower would determine if we add the difference to the runoff or subtract it.
If your runoff came out more acidic (for instance: 6.5 in and 5.0 out) the equation would be x = (6.5 - 5.0) - 5.0.
The idea is to think of the runoff pH as the middle point between the water in and the soil. So that if you put 6.5 water into 3.5 soil your runoff will be 5.0.
I ignore PPM's unless they are super high from the rest of the plants in the same run and they look ill. Usually this is a lockout due to high pH though. PPM's are also important if they are dropping. It means your ladies are running out of food. Other than those to things I don't worry about PPM's.
One of the growers I talk to out by me says that his PPM's came out 12K one time and it was the best yield he ever had. How much of that is true idk, but I do know that not everything that works in someone else's garden will work in yours. I just know this formula works in my garden.
The pH of the next feeding runs off of this formula. It grabs the pH of the soil (that was our "x") and then calculates the soil pH's opposite so that the two mixed equal 6.5 and then divides that difference by half, so as to gradually adjust the pH. If it's something absurd like 3.5; I'll be more aggressive
