From that height – 300 miles – LOS is pretty much all of the US and parts of Canada and Mexico, provided it is biffed somewhere around the center of the US.
First dieoff will be from stuff like airplanes going down, pacemakers stopping, folks on dialysis, etc. Second wave will be after the meds give out after a couple 3 or 4 weeks or so. Wave 3 will be sickness, accidents, etc.
If you can make it through the first waves and you have your wits about you, I feel you should be okay. For awhile. As was stated, the problem lies with our infrastructure. Not only will the grid be fried (and there is about a 3 year wait for new transformers), but the powerplants themselves will be cooked. Even if they weren’t cooked, how will you get fuel to them? Or transmit the power down the lines even if you could get fuel to them? The diesel electric trains used to haul coal to the powerplants will be fried. Rarefied medicines are made hundreds or thousands of miles away. No way to get them to the people who need them the most. Water treatment plants and pumps will just stop. Farmers might have an advantage in that they live where the food comes from, but they have no means to get the food to the people who need it – who might be clear on the other side of the country. And thanks to GMO crops, they can’t hold back a portion of their harvest to replant the following year. If they do, whatever sprouts up will be nonviable.
It’ll be run what ya brung time. 90% might be a good estimate, it might not. It’s never happened before anywhere on earth, so nobody really knows. The country will be tossed back a couple hundred years, but the people will still be stuck in 2014. When compared with all 300 million of us, those with the proper skill sets and resources held back will be few. So yeah, a 10% survival rate might be likely.
The wicked flee when none pursueth..." - Proverbs 28:1