There is a very tight number of targets to take out in either nation. You of course do not "wipe out the whole country", you take out: missile launch complexes, C3 centers, select airbases, select political-military command sites, select naval bases, select miscellaneous sites, select cities starting with any containing the above. The target list for the US beyond Washington wouldn't start with New York or Los Angeles, but with Denver, Houston or San Diego.
If you got a couple left over, dump particularly dirty stuff into the greater Los Angeles and New York area. Remember, about 25% of the US population lives just in those two metropolitan areas.
Source, offhand, not sure, but the "UNO Report on Nuclear Weapons" (book released in... 1983 iirc) would make a good start. Iirc that above is in there, as well as contamination spread examples for select targets, which in an applied spread pattern with 150 warheads or so in the right way would have affected 60-70% of the population.
Too lazy to go downstairs and look the book up right now though.