High SPF does not necessarily offer greater protection and it is only a measure of UVB protection. UVA also needs to be considered. Here is a good explanation.

http://www.skincancer.org/skin-cancer-information/ask-the-experts/does-a-higher-spf-sunscreen-always-protect-your-skin-better

SPF testing in the personal care industry is conducted with special instrumentation and the substrate is a roughened PMMA plate to better simulate real application on skin. While not perfect and results can vary between labs it is the industry standard.  Here is a link with information about the instrumentation used. 

https://www.labsphere.com/labsphere-products-solutions/components-accessories/spf-upf-testers/

thank you so much dear Kelly Dobos