OMFG!
pH of the run off is simply the pH of the run off! It is the most inaccurate way to attempt to figure pH.....
When you water the pH of the soil will drop and as it dries back out it will rise back up to it's actual soil pH value.....Be sure to have enough liming agent in the soil to start. FF soils need extra to start = add 1/2 cup of Dolomite lime to every bag of FFOF or FFHF..
If using synthetics - pH ingoing to 6.7 and water......soils under 6.5 tend to have poor Ca, Mg and P uptake....keep that in mind.
You do NOT have to water to run off at all - depending on what and how much/often you feed...
I would really be edgy about watering that much and letting my roots sit in wet soil as long as it would take to dry that out.....The more often you water the more o2 gets to the roots and that is VERY important.
What nutrient do you use?
How much do you use?
How often do you water? Feed?