You still have 4-5 weeks of flower growth to go, so nowhere near time to flush just yet.
When there are 95-98% brown pistils/hairs, you will be entering the harvest window.
At this stage you will need to keep feeding them, especially P and K, to get the flowers to bulk up.
"Flushing" is bit of a misleading word, you do not need to pour gallons of water through the pots all you need to do is stop all fertilizers and supplements and give just plain water in normal amount and at normal intervals and let the plant "flush" itself naturally of any built up nutrient salts.