To answer the question about what a reasonable number is, you do have to make a model of the conditions that are there. Basically a broad story about what was going on. Then find reference data to give you a baseline and then put corresponding data into your model. This document has some data that might help:
http://www.un.org/popin/popis/journa...today0295.html
Some highlights that may be of interests is the effect of the black plague on global population. From 8000 BCE, the dawn of agriculture, to 1 CE, the global population went from about 5 million to 300 million in those 8000 years, or 0.05% annually. From 1 CE to 1650 CE, the population grew to about 500 million, much of the low rate caused by the black plague, which was not limited to the 14th century, but is thought to have started in Western Europe around 540 CE and spread. The growth rate from 1 CE to 1650 CE is lower than the prior 8000 years.
These eras are significant since they are periods where agriculture is starting to come into use and where there was a highly fatal disease. Both of these are conditions that are happening after the war. The growth numbers after the war might not be quite as low due to education and limited residual tech, but that advantage would drop quickly as machinery breaks and knowledge gets lost.
I don't have specific numbers for you, but this should help you get on the path.