What is of interest is not the blocking percentage, but the transmission percentage.

According to WikiPedia:

"For example, "SPF 15" means that 1⁄15 of the burning radiation will reach the skin, assuming sunscreen is applied evenly at a thick dosage of 2 milligrams per square centimeter[67] (mg/cm2)."

so assuming a linear dose response relationship (obviously oversimplified) when not using the sunscreen 15 times more instantaneous random damage is incurred compared to when using the sunscreen.

This does not translate directly into the rate of cancers though: just like the final damage of a meteorite storm isn't proportional, even though the instantaneous damage is.

Suppose a meteorite strikes a hospital, lots of damage. Then years later a meteorite strikes a school, lots of damage. Obviously if both happen in quick succession more damage will occur.

But if the whole human population takes up sunscreen use, selective pressure on cellular coping mechanisms will be relaxed, and eventually future generations won't be as resilient against sunburn. So just live your life, and don't allow scaremongers to separate you from your money, or thus indirectly scare you into doing your job for them.