I usually chop when about 15% of the trichs are amber. I've grown most of my strains long enough that I have a good idea how long they take to mature, and flush accordingly. But if you're wanting to chop at 10%-20% amber, then start flushing after all of the trichs have turned cloudy, and no later than when you begin to notice a few turning red. Again, in my opinion, best way to know is to have a few runs with a particular strain under my belt, which gives me a much better idea on my flowering time.

Hope that helps. Take care. :jointsmile: