when to flush

Don't flush. It doesn't change the makeup of the plant or the harshness of the smoke. Harshness comes from bad curing. People prefer the taste of unflushed bud. That's where the science is right now with it. If you must then flush around 7-8 weeks into the flower cycle

it depends on when you're going to harvest. I'd flush 1-2 weeks before you plan to harvest your plant. If you're seeing trichomes starting to amber, I'd say that's a good time to start your flush, so by the time 30% of your trichomes have ambered, you won't have any nasty nutrients in your plant that can cause harsh smoke. Enjoy your crop!