Goldenrivett, I'm not sure if you misread the original post or are being uberprecise. mstram's DTO was to where he thought the magnetic pole is, and that largely means that the true pole and magnetic variation don't enter into it. But I made an allowance for uberprecision too when I said "0 degrees give or take", as the nonuniformity of the earth's magnetic field will likely cause the plotted course to vary from exactly 0 degrees magnetic.
Or to put it another way, the GPS will plot a great circle track to the magnetic pole. If the magnetic field were uniform then its true bearing would be exactly the negative of the variation no matter where he started, and converting it to magnetic would yield exactly zero. But since the field is not uniform his result would be zero, give or take.
However you think about it, 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.