How Much Fragrance Do I Add?

I saw someone ask this on Instagram. How do I know how much fragrance to use? It is not straightforward, and it can be confusing. So, I wanted to talk about considerations for fragrance use. This post assumes that you do want to use fragrance in a formulation. I do have a post on if you should add fragrance in the first place (Link). I will also be talking about both essential oils and fragrance oils.