Good indicator for time to flush?

You still have roughly 3 weeks to go, so don't get too excited yet. There is no need to drown your plants with gallons of water to "flush" them, this does nothing special. Just stop all nutrients/fertilizers 10-14 days before harvest and give plain water in normal amounts and at normal intervals, and the plant will "flush" itself naturally. For me, this means stopping feeding when there are about 95% brown pistils and harvesting when there are about 98-99% brown pistils.

you should just give them water from now on but actual "flushing" is not needed in my opinion sure you may be removing excess salts from the medium but its not doing anything to the plant itself people believe that flushing will strip excess salts etc from the plant itself however it does not work like that . if you want the plant to use up extra nutrients then just feed water and the plant will take what it requires via mobile nutrients from the leaves themselves