That might indeed be a “rule of thumb” but it’s coarse at best.
FWIW when I spent a year logging this a typical Europe to Eastern USA flight was “worth” typically 50 -60 micro sieverts, west coast US into Europe over Hudson’s Bay might be 80 plus on a bad day, but it was highly highly dependant on flight levels used , magnetic latitudes and the Sun’s output on the day ( and as an aside FWIW during solar max our exposure to cosmic rays is reduced, though we may be more exposed to the Sun’s own products, so it is all very much non linear).
CARI factors the above in it’s calculations, and a full time, full “spectrum” measuring device might be even more useful.....so it’s fairly straightforward to get a rough handle on the amount we get exposed to....but as for effects on health, well as a doc said to me a while back about the incidence of a particular cancer, and I paraphrase, “like most cancers it’s more than likely down to your relatives rather than your job...”.