Harvest when there are 90-95% brown pistils for a mind bending THC influenced head high.
Harvest when there are 98-100% brown pistils for a heavier and sleepier CBD influenced body type of stone.
Let your preferred type of effect guide your harvest timing - head high or body stone.
Start flushing 7-10 days before the desired pistil colour is reached, by using just plain water, nothing else.
There is no need to drown your plants, just water as normal, but only use plain water, nothing else, and this is all that is required for a suitable "flush".
"Flushing" is basically about reducing/removing the amount of nutrient salts in the plants' cells, it is not about drowning the roots or soil.
Hope this helps, Organoman.