Hi SM,

One thing to keep in mind about this cookbook.
F = I x B sin(theta)



where,
F = force in Newtons
I = current in Amps
x = length of tether in meters
B = Earth's magnetic field strength in Teslas
theta = angle of tether wrt magnetic field lines in degrees



This is a simplified case for the original equation from which it stems.

F = I X B

...where F, I , and B are 3D vectors and X is the cross product operation.

It only works if I, B and theta all remain constant for the length of the tether. For a reasonably long tether, B and theta will not remain constant in the dipole field of the earth.

I figure this will decrease in strength by the square of the distance from the center of the earth.

It actually decreases as the inverse cube (1/r^3) if you assume a dipole source at the Earths center, which isn't a bad place to start. A better approximation is a loop of wire an arbitrary distance under the Earths surface but that for anything off axis is MUCH harder to solve for.

This is just one part of the whole package. The other and more dificult will be the electrodynamic aspects of draging a conductor of that scale through a changing magnetic field which will generate a significant potential (voltage) along the tether, which you may have to work with or against.