dunno .. but I keep bumping in to thread like this .. wher Ppl. use it and have problems with over feeding ..
if I wher to use FFOF soil I would atlest mix it up with +30% coco fibers (neutral coco fibers with no nutriens added)
and yea a good flush with some 6,5-7 PH water might help make sure you see a lot of runn off ..
and sure you can take a PH test of your runn off .. but do it with a "grain of salt" so to speak dont think it will come out the same ..
if you water with 6,5 -7PH (as you schould in soil) it will mostelikely come out ½-1 point lower .. depending on how much nutriens/salt you keep in the soil ..
but I use it to keep a eye on my girls ..
I know if I water with 6,5 (light green in the test) it will come out around 6 (yellow in the test) if I see it come out way off I make a cheap soil test .. just to see if Im totaly out of ranges