If you want to flush by the EC, then flush until the EC out is not too much higher than the EC in, and at the correct pH (2nd link in my sig I think, the chemistry one), and then feed IMMEDIATELY at 1/4 strength . Next feeding, go back to your normal feed schedule and then start a regular pre-harvest flush one week from choppytime if you feel you need it. That's your call, but I'd do at least just plain water the last week even if you don't do a final flush.