See Mason. Barring that, see my previous post in another thread on washing which gives the equations and data from Mason. The article that Lee refers to is correct to a point. The amount of hypo approaches an equillibrium value, but that value is far lower with flowing fresh water according to the same type of math as in the article.
You see, the bath has to be sufficiently large, and by that time you are sometimes passing the break even point in water conservation.
Well, read Mason or the other post.