I believe the US government used to keep food reserves expected to feed the entire US population for 18 months. Given that they sold off the last of those reserves in 2008 except for some butter and cheese, and given the average household and grocery store only has 3 days of food on hand, I think that 90% death rate in a grid down situation will be fairly accurate. Once you get to that few people the government and corporate elites will likely start caring about the need to keep some of us alive to scrub their toilets and fight their wars.
Life was actually pretty good for the lucky ones who survived the Black Death years. Lots of land and resources available.. And with a very limited available labor force the people began to have more say in how they wanted to live their lives. Though the potential rebound from a grid down disaster could be severely impacted if the response to a HEMP or grid destroying cyber attack leads to WW3. I’m not sure I would want to live through a nuclear winter even if I had the incredible amount of resources needed to do it.