Hello guys,
just made a graph with data from cari-6 which is a nasa program to calculate inflight radiation.
It should be self explanatory, but in the bottom right I made an example to help you understand:
if you cruise for 1 hour at 35000ft you get a nice 3.86 uSv of radiation
if you do the same at 41000ft you get +40% of that!
Hope this helps you to make an INFORMED decision next time you have to decide if you want to save fuel!