You should absolutely experiment with the optimal pH of your water if you can. Keep in mind that the other ingredients have different pH levels, so even if you use water that's 4.5-5.5, you'll still have to test the final product's pH and adjust it. You may find that using a different pH level works even better. In terms of quantities, I recommend going with the ones listed in the Beginner's Guide to Making Haircare at Home.