I prefer to make this recipe in small batches and use it up quickly, which is pretty easy since my hair is super thick and my daughter’s is nearly to her waist. However, it is possible to add a natural preservative such as Optiphen Plus to extend the shelf life. Usually formulators add it by weighing the total amount of liquid (in this case about 50 grams), then multiplying that number by the recommended percentage of preservative, which for Optiphen Plus is from .75 to 1.5. The resulting number represents the total weight of the Optiphen Plus liquid that should be used. In this case, that would be between .38 grams or .75 grams. Deciding how much to use and whether a particular preservative is the right fit for a formula is not just a matter of percentages, though. A lot depends on the pH of your formula, which can vary based on the pH of the water in your area.