OK.

So on your account it follows that if N% of the U.S. population comprises preppers, then after a nuclear event we should expect to see ~1.4B "non-modern" people, (.005*[1-N]*us_pop) non-prepping USAers, and (.75*N*us_pop) prepping USAers, among others. After some other scenarios we should expect to see ~1.4B "non-modern" people, (.2*[1-N]*us_pop) non-prepping USAers, and (.95*N*us_pop) prepping USAers, among others.

Yes?

So, OK. If us_pop is 315486161 and N is 0% then in the first scenario we expect the survivors to include (.005*1*315486161=) ~1.6 million USAers and in the second scenario we expect the survivors to include (.2*1*315486161=) 63 million USAers.

At the other extreme, if N is 100%, then in the first scenario we expect the survivors to include (.75*1*us_pop=) 237 million USAers and in the second scenario we expect the survivors to include (.95*1*us_pop) 300 million USAers.

In all of these scenarios we also expect the survivors to include ~1.4B non-"modern" people, plus some modern survivors not from the U.S.

Yes?

Given those estimates, and assuming that the referents for your "prepper" and JMIV's "crazy survivalist" are roughly comparable, I don't find myself caring very much about Ns smaller than about 1%.