I've flown with a pilot who had his own geiger counter on a trip to Australia. Taking regular readings in the climb and cruise, it seems that each 4000' increase in altitude roughly up to doubles radiation exposure in the 30k' levels. Heavily dependant on latitude and other factors. To chart radiation levels for the world at altitude would lead to charts as complicated as isobaric (lines of magnetic force?) charts. I ws surprised that the geiger counter going off every 6 seconds at ground level was reading c 360/minute at cruise altitude. Flying 1/10 year per year, you are getting more radiation than nuclear power station workers and you are just within the legal limits for radiation.