Do it yourself failure

I decided to try to make my own shampoo using this recipe. I'm sure that it is time tested and all that, but my problem is that I don't really have a concept of how much a regular shampoo bottle can hold. It turns out it can hold 2 cups minus a little bit, but the recipe makes 2 cups plus a little bit. Consequently, I was unable to stir it up well. I tried it out in the shower this afternoon and it still seemed to work well though it didn't lather up much. I think next time I'll make just half the recipe.