The amount used actually depends more on the type of water you have and how dirty the clothes are. You can add as much as you want, but you don't need to, so why would you waste it. I wasn't aware that soap flakes were still sold in the US; several companies used to make them, but have quit over a decade ago. I wondered if any ecologically-safe companies would bring them back for laundry. The cheapest way is to just grate bar soap--either in a blender, processor, or by hand, depending on how fine you want it. The really fine grate works best in the powdered laundry recipe, since it dissolves quickest. While the original recipe calls for fels naptha, any bar soap will work. My grandfather used to make and grate home-made soap for his laundry, and I have used it too. By the way, if you do use homemade bar soap, the liquid recipe will stay liquid without stirring since homemade bar soap doesn't have any additives to make it set up like commercially-made bar soaps do. You can even just use grated soap if you want. The borax helps clean and brighten the clothes; the washing soda helps clean stains. If you choose to add baking soda, it will help soften hard water. Some add plain table salt for its cleaning effect, its ability to set colors, and to keep them from fading. I've even read reports that clothes (that aren't greasy) will come just as clean washed in plain water with no soap added. Like any other homemade product, there is a curve for what works best in your situation. As you have read, lots of us have found what works for us, hope you find what works for you.