Razza, heres what I did. I was in a similar situation as you. I use promix. On day 1 of flush i took them into the bathroom and in the tub, i flushed them with about 2-3 gallons (2g pots) of water. I figured this rinsed all the fert out of the soilless mix. Then, for the remaining dys, i just watered them with a little more than usual amount of water. So, really, i only dealt with runoff once. Who knows if this is the right way, but my buds tasted just fine.