centimeter of skin. That’s approximately a full finger length of product for the face and another for the neck. I don’t know anyone that uses that much foundation! So, let’s say your foundation has an SPF15 and you’re using, say, half a finger length of the product. The amount of protection doesn’t just reduce by 50% but likely drops to around SPF4.
Think of it as a safety net, for example, if you apply your broad-spectrum SPF, followed by a tinted moisturiser with SPF on top = you’ve created an extra filter for the UV to penetrate and thus increasing your protection. This is good, any extra protection is welcome, especially if you aren’t great at topping up your SPF throughout the day.