Strength of magnetic fields

I've been given the question to compute the strength in the van allen belts when given the strength of Earth's magnetic field at the surface. Then, I am supposed to calculate the gyroradius of a 50 MeV proton from the strengths I come up with. I want to know if I am going about this correctly.

Is it ok to use say that B = B (at Earth's surface) * (1r (Earth)/ r (belt radius in terms of Earth radius) ^3

For example, if B at surface is 4 *10^-5 T and inner belt radius = 1.5 (earth radii) then I will get the equation:

B = 4 *10^-5(1/1.5)^3 = 1.185 * 10^-5 T

As for the gyroradius, the formula is r = (mv/qB)--can I go ahead and assume the velocity of the proton is equal to the speed of light. The notes that I am using to carry out this problem does not specify any way of getting the velocity of a particle moving through the belts.

Please let me know if this looks ok! Any help would be appreciated. Thanks!!