I've done many, many flushes out of moon-sync that I consider successful, meaning I got lots of stones out. Not every cleanse will be optimal, probably, but something is better than nothing and I'm very, very pleased with my results. Lots of people flush several times a week or month. Hope this helps.
Andreas later changed his opinion, saying that the flush will be successful if you BELIEVE it will be successful, regardless of when you do it. He wrote something on this on his forum, but I can't find the post.