How to calculate the magnitude of magnetic field of an orbital?
Hyperphysics means that the energy difference between the 2p J=3/2 state and the 2p J=1/2 state is as if the magnetic moment of the electron were aligned with and against a magnetic field of 0.4T.

The energy of the two states, relative to the ground state, is (according to NIST)

2p J=3/2 : 10.19885142904 eV
2p J=1/2 : 10.19880606470 eV

The difference is 0.00004538204 eV or $7.271004 \times 10^{-24}$ J 

The electron magnetic moment is $−9.28476462 \times 10^{-24}$ J/T

So the energies are as if they are raised and lowered by being in a field of 0.3915556 T.

For the ground state, 1s, there is no orbital angular momentum.  The 1s state is only split by interaction with the proton.  This is sometimes referred to as hyperfine structure. The energy difference is $5.87433 \times 10^{-6}$ eV.