Once you see cloudy trichromes about 50% start flushing, when they start to turn dark it's time to harvest. The trichromes that are turning dark is the signal that the plant is decaying, all dark is not great for smoking and in a lot of growers opinions it's junk.
As far as flushing goes I would switch to water and 20% food. Why add food when flushing???? First you plant still needs food for the next 2 weeks and it will use it to put weight on. The second and most important is flushing removes some minerals and micro nutrients fast than others, that's the big problem with just flushing with water. For example nitrogen is easy to flush out and one of the first to go, but if it goes to fast you lock the magnesium up in the plant. Does not smoke so well, ever smoke stuff that sparked or popped, yup mag lock out. (popping could be spider mites)