I dont flush so its easier for me.. When i see calyxes are swollen and hairs have receeded then i start checking trichomes. I taper nutrients down.
You never really know how long those trichs are going to take to go to your prefered ratio of cloudy/amber. But if you really want to flush i would do so when the recession of hairs and swelling has finished. If you know the strain ( you've run it a few times) then obviously you get a better idea of that ripening window.