Lo que es claro es que es un indicador "relativo":
"All this means that a sunscreen’s SPF is more an indication of how well you could be protected, rather than an iron-clad guarantee".
Esta es la "fórmula" para obtenerlo:
"To get the SPF number, a simple formula is used. The number of seconds it takes a patch of skin to slightly redden when covered in sunscreen is divided by the number of seconds it takes to slightly redden when there is no sunscreen applied. Say it took 300 seconds for skin to burn with sunscreen, and 10 seconds to burn without it. 300 is divided by 10, which is 30. The SPF is 30."