In both cases though the level of UV will be easily tolerated, which is the entire point. UV index is a linear scale, so more SPF has rapidly diminishing return

In both cases though the level of UV will be easily tolerated, which is the entire point. UV index is a linear scale, so more SPF has rapidly diminishing returns even in places with a UV index of 15+. That the duration of protection is independent of SPF makes this particularly true. There are only a handful of places in the world where atmospheric conditions might give a very high SPF marginal benefits.