It is best to include your preservative as part of the original formulation, as a percentage, by weight. This calculator adds the preservative on top of an existing formula, which isn’t ideal, but is a decent place to start. This will result in a slightly lower usage rate, but if you’re using the maximum recommended amount that should still be within the recommended range. For instance, if you calculate a 100g recipe at 0.5% that’ll tell you to add 0.5g preservative, which means your batch size is now 100.5g. The preservative is now technically present at 0.497%, which is pretty darn close to 0.5% and within the recommended usage range.
Remember, there is always more to preservatives than just the usage rate! Be sure to look at effective pH range, solubility, and anything other conflicts. For relatively foolproof preserving I recommend using Optiphen Plus as it has a broad pH range, is water soluble, and you won’t easily de-activate it. You should use Liquid Optiphen Plus at 0.5%; the recommended range is 0.1–0.5%, but since our home laboratories are nowhere near sterile, it’s best to err on the side of more. Don’t use more than 0.5%, though—more preservative is not better!
If you want to use something else, please check our other preservative product pages in details. That will give you the recommended usage rates and details some incompatibilities or conflicts.