The largest magnetic deviations that I know of are, in the northern hemisphere, near the north magnetic pole. Across the United States the magnetic deviation is between 31' east and 31' west, more or less symmetric about longitude 100 West. As you get up towards the higher geographic latitudes near 70 - 85 degrees and longitudes of 110 degrees west, in the Northwest Territories, deviations as large as 80 to 90 DEGREES can be found, and this changes very rapidly as you move around in the arctic regions. You can study this yourself by entering geographic locations in the Canadian Magnetic Reference Field Modern.