A tablespoon! I have wondered about the detergent being fully rinsed out with my new washing machine. It spins for a very long time but uses a lot less water than the old one. This would have no effect on rinsing out the detergent. I will use less each time to see how little will still do the job.
Well, we have been using that amount for several years now and clothes get clean. The savings on detergent are phenomenal.
Hope it works for you.