Brian,
I've converted over the "coremag" mag routine, which is called by the 15-State with magnetometer EKF. The 15-State uses the field[3] to field[5] values in the update stage. These three values are supposed to be the "local magnetic variation given position, altitude, and date." By any chance can you tell me if the numbers below seem about the right size, i.e. order of magnitude? I'm still searching the references to see if I can find that out, but no luck thus far.

JD is 2440553
field[0]: -45383.26
field[1]: -25598.36
field[2]: 3398.72
field[3]: 25728.97
field[4]: 3398.72
field[5]: 45309.34

Don