Thickness of low frequency Mu shield calculation
These came from a textbook somewhere, but I can't remember the name of the book:

Absorbtion losses (in dB):
\begin{equation}
A = 20 \frac{t}{\delta} \log_{10}(e)
\end{equation}
t is the material thickness, e is the base of the natural logarithm, and \$\delta\$ is the skin depth.
\begin{equation}
\delta = \sqrt{\frac{1}{\pi \omega \mu \sigma}}
\end{equation}
\$\omega\$ is the frequency (in rad/s), \$\mu\$ is the magnetic permeability of the material, and \$\sigma\$ is the resistivity.

There are also reflective losses:
\begin{equation}
\|Z_s\| = \sqrt{\frac{2 \pi f \mu}{\sigma}}
\end{equation}
Then magnetic reflective losses (in dB) are:
\begin{equation}
R_m = 20 \log_{10}\left(\frac{\pi f \mu r}{2 \|Z_s\|}\right)
\end{equation}
Where r is the distance from the source to the shielding and f is the frequency in Hz.

I'm sure there's some assumptions made here, but I don't know what they are. Total attenuation is the sum of the two.