i go by if its 50% u MIGHT need a flush soon (some strains sooner, sum longer) but thats my lil rule of thumb....but ive only handle 2 strains, 1 ALOT more than the other. but the 1 came out nice the second time around. wen i did a GOOD flush let it dry (not fuly dry, dont dry ur roots out) and another flush, and supplamented feedings with Humboldt nute HONEY, so they r still eating. (HONEY es is basicaly 'blackstrap mollasis' with a couple other goodies in it)that way they NEVER loss the color in the leafes and they stay dencing fer the last couple weeks of it life (thats wen it REALLY packs the WEIGHT on, not so much size, but GOOD weight)