spf and powder products

I was wondering how effective powder SPF products are, so did some searches.

According to this link, you'd have to use a lot of powder to get the stated SPF!

When scientists test facial powders to determine SPF (as mandated by the FDA), they typically assume that 2mg of product will be used per cm2 of skin. The average face is about 600cm2 (although that varies from person to person, of course), meaning that a person needs to apply about 1.2g of facial powder to get the SPF stated on the product's label

It's funny how unclear the details of SPF are.

http://en.wikipedia.org/wiki/Sunscreen#Sun_protection_factor

http://www.fda.gov/cder/offices/OTC/understanding_spf_sunscreen.htm

http://www.slate.com/id/2171789/
http://health.howstuffworks.com/spf.htm/printable
The above two articles have interesting similarities...