I'm not sure I agree with your numbers there.
I know they aren't right but it was late and I was on my way to bed. I have the actual article stating the value here somewhere in my room but as it is immaterial I haven't been bothered to find it.
I think it said that a particular species of fly (I can't remember exactly which one) could survive a dose of up to and not including 10 million siverts (10,000,000Sv).
You want to be looking as possible non-terrestrial environments, and quite a lot of those have lower background radiation levels than Earth.
You are absolutely right that the Earth has a relatively thin atmosphere and that we sit on the inner half of our sun's goldilocks zone. In that sense the Earth has a relatively high amount of celestial radiation but have you thought about the radiation that the planet itself will generate?
The interactions between the layers of a Gas Giant, its rings and its moons are bound to create massive electromagnetic distortions and possibly generate a lot of radiation.
Never-the-less you are correct to say that there will be organisms with far lower and for that matter far higher radiation resistances.
Radiation resistance... This must cost something to raise else everyone will instantly go for high.
I was contemplating a way to make radiation resistance an automated system that the player has no direct control over and I realised that it all depends on how radiation levels are generated for planets.
I would think it something like this:
-The thicker the atmosphere the less radiation.
-The further from the star/stars the less radiation (Inverse square law).
-The less radioactive resource produced the less radiation.
As such the radiation resistance of a race could be set up to follow the same rules.
-Atmosphere type has an effect. No atmosphere has highest radiation resistance, really dense atmosphere (this is were air pressure comes in at least as a background, non active, calculation aid) has a really low radiation level.
-Racial types have different effects. Say a Humanoid has a moderate and an Insectoid a high resistance.
-Home Planet type has an effect. A gas Giant has less radiation that a terrestrial.
These are all determined during race creation but a second set of influencing factors could be added so that the same race has a different resistance every game.
-Distance of home-world from star. The further the lower the resistance.
-Radioactive resource output of planet. The higher the output the higher the races radiation resistance.
-Number of stars. If you start in a system with three stars the radiation output will be aprox 3 times greater so your resistance will be aprox 3 times greater.
What do you both think?