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.

True, so the important factor is - how does this map to your chances of getting skin-cancer?

The most reasonable answer is to look at the transmission percentage, not the blocking percentage.

No it isnt. Two photons is twice as much transmission than one photon, but both cases are totally insignificant.

How? Sorry I'm confused by that statement.

If you have a "100 unit" light bulb, and a material that blocks 98% of the light it emits, 2 units of light are getting through. If you have a material that blocks 99% of the light, only 1 unit - half as much - is getting through.

(This is why the SPF scale is inverted/measures transmittance. SPF 50 sunscreen theoretically allows through 1/50th of the UVB radiation (or whatever wavelengths are specified by your local regulator).)

98% = 2 units of UV reaching the skin

99% = 1 unit of UV reaching the skin

Thus 98% filtering lets in 2x as much as 99% filtering