m39462,
the only reason he saw any significant drift from zero as he sallied westward was that his plotted destination was several hundred miles away from the actual magnetic pole.
If you read Checks Complete's explanation more carefully, his statement that the 'Rules of Thumb' are a guide and work reasonably well in the local area but once you look at long distances there are too many variables for them to work." is correct.
The "rule of 60" which the OP applied is approximate and only works on a 2 dimensional flat surface. With large distances (e.g. over 5,000 miles) spherical trig is necessary as the curvature of the earth's surface is no where near a flat plane. Even if the correct position of the Magnetic Pole had been entered correctly, "the rule of 60" would still not produce the correct result.