Assuming an intentional EMP sent America back to the stone age, then America would respond in kind to China, N.Korea, Iran, Pakistan and Russia. England, continental Europe, Turkey, India, Israel, Japan, S. Korea would all have been hit with the same at the same time. If somehow we all just traded queens and stopped there, or if a prolonged solar storm hit the planet repeatedly accomplishing the same, I really think your optimistic limiting global losses to 75%

After the first northern winter I would assume 90-95% of humanity is gone. Those that can hunt are the most likely to survive. Seafood increases your odds of survival by orders of magnitude, as it requires some skill, but much less caloric investment before reward. By and large we've lost the ability to farm without fertilizers and pesticides, many will try but one early freeze will kill off dependant communities. Climate change making weather less predictable def does not work in our favor.

After the first winter I wouldn't be afraid of strangers. Another person is far too valuable, when you remember that there are lots of things outside that will happily kill us. We only have strength when we're in numbers.