And in 90% of cases 30 is sufficient, unless you are going to the middle east, sub-Saharan countries or Australia mid summer you only need 30.
Edit: On a scale from 1-10, 1 being a ginger and 10 being an Ethiopian, my skin tone is a strong 2 or a very weak 3 and I've only been sunburned in Dubai whilst using sun factor 30
Most of my friends / family who are easily a 4 just use s.f. 10-15 even in 40+c summer weather