Soil growers don't often use PPM meters. We usually just use measurements as our fertilizer levels don't need to be anywhere near as exact as with hydro. It would, of course, be very useful though.
After flushing, you want to let the soil dry extra well before watering again because flushing is overwatering; the beginning of it, at least. You'll also want to feed them extra well the next watering because the flushing washed all of their nutrients away.
Your nutrient solution- how old is it? Organic ferts don't usually stay good for more than a few days unrefrigerated, once mixed. However you adjust your ferts, add more if you dilute them. As we said, the flushed plants will need full strength food next watering. You can just add the proper pH adjuster to a fertilizer mix without adding more water.