— Figure 29-5 from Cosmetic Dermatology: Principles and Practice, 2nd Edition by Leslie Baumann
All products with an SPF or UVAPF are tested at the exact same amount, 2 milligrams per each square centimetre of skin (2mg/cm2). If you’re just dusting on your foundation or powder with SPF, to matte the skin or provide minimal coverage, you’re not getting the SPF/UVAPF on the label.
I’d recommend that you use a sunscreen (about ½ a teaspoon for the face and neck), let it dry (about 15 minutes), and then apply your powder for best protection.
There are newer sunscreens that contain primer ingredients like silicones and hydrogenated polydecene built-in.
P.S. Where’s the ½ a teaspoon for the face and neck recommendation coming from? Estimations – two of them. The first estimation is the density of an average sunscreen, and the second estimation is the average area of a face. It’s not accurate for everyone, but the exact amount (2mg/cm2) is difficult to translate!