What I do, is write down the % of each ingredient that I know won’t change. For example, the emulsifier, or active ingredients, and preservative. Then, I add the oils in the concentration I want depending on texture, and fill the rest with water.
For example, for a lotion, I would maybe use:
1.5% preservative
5% active ingredient
5% emulsifier
25% oils
63.6% water or other water-soluble liquid
Then, if I want 543g of lotion, I just do 1.5% of 543g = 8.15g preservative, etc.
If you are using an already made recipe, like those from Marie’s blog, just change the water % to allow the preservative. Ie. If she uses 70% water and 0.5% preservative, but yours uses 2%, then use 68.5% water and 2% preservative to still get 100%.
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