As fibre/ bulk is proported to help build pressure in your liver making the flush more productive, I would say to do one before the juice fast. Personally the flushes wear me out a bit as does fasting so I wouldn't do them at the same time. You will very likely need more than one flush, some people do as many as 20 or more before they stop seeing stones. I found it helpful to pace myself and not do too much at once. Remember your kidney has to filter all these toxins so go easy on it. Your Naturopath will know best.