duck and cover is actually fine for ordinary atomic weapons in the kiloton range. If you don't get fried the major source of injury is flying debris.
But not so much for thermonuclear megaton weapons where most everyone in the target area gets fried anyhow.
Maybe I'm not smart enough to get your differentiation because it seems you're saying the victim's safety depends on range of the weapon in relation to the civilian target.
back before the invention of thermonuclear weapons, duck and cover was sensible.
once thermo nukes were common not so much.
This was a period of about a dozen years from the first atomic bomb before thermo nuclear weapons were deployed on a practical basis
http://www.weirduniverse.net/images/uploads/picatomic_thumb.jpg
https://i.pinimg.com/originals/89/c5/ef/89c5ef119c4e67fd49d1895a0dae9ed4.jpg
https://i.pinimg.com/originals/e6/9b/f7/e69bf7ad0a3e6c9985b8ebe13a2a77c2.jpg
after which point we get this sort of thing.
https://prophecypanicbutton.files.wordpress.com/2012/06/basilnuclear.png
This reminds me of the Duck and Cover drill the government enacted for students in early days of the Cold War.
It was as useless as a bucket of rocks ( http://wnep.com/2018/03/22/superintendent-says-students-are-armed-with-rocks-in-case-of-a-school-shooting/ ) but was supposed to make kids feel more secure.
The teacher locks the door and turns off the lights so the actual safety part for children (elem. age by the looks of the pic) is to hide, which is a normal reaction. This instructional sign probably adds to terror more than not having it posted.