Well if it's any help LT, the initial weapons test that highlighted EMP, Op: Starfish Prime, was done at an altitude of 400km/250miles above Johnston Island and some 1450km/900miles from Hawaii and it caused damage to the electric grid there - the height and therefore azimuth, is particularly important for the spread of any EM pulse (and that goes further to take into account such things as ground clutter & reflection etc. etc.)
Here's links to a few articles meant for the general public, they have little hard info but still contain some useful points: -
http://www.vice.com/read/we-asked-a-...emp-attack-508
http://channel.nationalgeographic.co...es/emp-attack/
This link has a better breakdown: -
http://www.forbes.com/sites/peterdet...ivial-someday/
If you want some "light" reading (oh yes that's a pun on several levels!), then the following pdf has a more in-depth study of the Starfish Prime explosion and it's effects on Hawaii: -
http://ece-research.unm.edu/summa//notes/SDAN/0031.pdf