You really cannot use US population increases to determine its growth rate, too many immigrants skewing the rate. You have to use world population growth rates which eliminates the immigration variable. Only births minus deaths (from all causes - war, disease, etc.) determines the growth rate.
It is estimated that the total polulation of the world reached one billion in 1800. By 1927 the world reached two billion. This was a time where there was some pre-industrial farming, the establishment of industrialized processing of food, and the beginnings of industrailized farming with the steam tractor on the Great Plains of the US. I determined that this growth rate was about 0.55% for the time period. This covers the time period associated with tech levels F and E in 4th edition.
From 1927 to 1987, a time of great advances in farming, industrial processes, medicine, refrigeration and freezing of foods on a industrial scale and appliances for the consumer, the population increased from two billion to five billion. This required a growth rate of 1.52% to achieve. This covers the time period associated with tech level D, C, and B in 4th edition.
The growth rate of 0.9% is approximately the same rate of growth of the world population from 1800 to 1987. About half of this rate 0.45% (actually it is 0.4621%) doubles the starting population in 150 years. According to 4th edition there are 20 million people living in North America 150years after the war. About 13.5 million live in the area once claimed by the US based on 2015 populations percentages of the US, Mexico, and Canada. In general the growth would be, on average, uniform across North America IMO.
At the end of the war only about 2.3-2.5% of the population survived. The first 50 years the growth might have been 0.2%, the second 50 years the rate was 0.4% and the last 50 years the rate has jumped to 0.8%. Of course, this is adjusted to local communities tech levels and such.
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