Do you want a powder, a gel or a cream? I make cream and I use 12 oz of soap, grated to 16 ounces of washing soda, to 16 ounces of borax. Dissolve the soap in 96 ounces of water (boil and stir). Cool to about 100 degrees and add in borax stir till dissolved then add washing soda stir until dissolved. You can add fragrance at this point if you wish. Stir once more and leave it to cool completely (overnight for me). When it is cool it will be a gel. Take a stick blender to it to form a cream. One tablespoon of cream per load of normally soiled clothes. If you keep it gel you will use a bit less. I'm not sure of how much powder you would use but this recipe yields me about 4 and 1/2 quarts (one quart is 64 loads). So a total of 288 loads. I start with 44oz of dry ingredients so that means I use less than .2 ounces of dry ingredients per load. Of course I think the water added helps to better disperse the ingredients for washing.
I cut up a 5 oz bar of soap into my food processor and mix to a fine powder. Then I blend in 1 cup borax, 1 cup washing soda, and 1 cup Oxyclean. Use 1 Tbsp for front load washing machines or 2 Tbsp for top load washing machines. My DH doesn't have any allergic reactions to it and it cleans better than store bought laundry soap. When you use it, always run hot water first to make sure it is fully dissolved. Then switch your water to the desired temperature and add your clothes.