Agreed, I can't know from first principles if the right shift length is 4 or 8 or 30. I'm sympathetic to your suggestion that it would be a surprising coinciden

Agreed, I can't know from first principles if the right shift length is 4 or 8 or 30. I'm sympathetic to your suggestion that it would be a surprising coincidence if doctors landed on the perfect number through historical accident. Maybe they have a feel for safety in shift length, but they don't in other areas like washing, so maybe not. It probably varies by specialty and even individuals, probably a monster of a problem.