Nice plants.
I always wait until there are about 95% brown pistils before starting with flushing and then harvest when there are 98-99% brown pistils.
This puts me in the "10-30% amber trichome" zone without having to look too hard and works for me.
I never drown my plants with gallons of water through the soil to "flush", I just stop with the nutrients for the last 2 weeks or so before harvesting and continue with plain water in usual amounts and at the usual intervals as the "flush". Again, this works for me.
Everything I grow is purely organic however. (no prizes for guessing that though!)
Cheers.