The topic of how much sunscreen to apply varies depending on which authority you ask and it can be very confusing.
We found a great video online made by a cosmetic chemist that provides a very detailed explanation of how much sunscreen you need to apply to get the SPF claim on a sunscreen based on how much sunscreen is used in the FDA required testing (2 mg of product per square centimeter). See video here: https://www.youtube.com/watch?v=NKS5kMbZBlk
In the video, the cosmetic chemist explains the following quantities of sunscreen are needed to achieve the claimed SPF:
Face only - approx. 61% of 1/4 tsp
Face + under chin - approx. 73% of 1/4 tsp
Neck - 23% of 1/4 tsp
Decollete - 45% of 1/4 tsp.
NOTE: She also explains that you can apply about 1/5 less than these amounts when you using a mineral sunscreen that contains a large amount of zinc oxide like Suntegrity because it is more dense.