Frequency effects on screening effect
For magnetic field screening, a non magnetic screen can be used effectively but the thickness of the metal screen has to be much more at lower frequencies than higher frequencies. This is due to the skin effect. For various metals the following graph shows skin depth versus frequency: -



So if you have a sheet of copper that is 1 mm thick and you apply a 50 Hz alternating magnetic field to one side, you will get significant field transparency because ideally you would need to have a thickness in excess of 10 mm.

This is related to skin depth \$\delta\$ in the vertical axis on the graph. Materials with a smaller skin depth for a given frequency are better at blocking the penetrating effects of a magnetic field.

For electric field screening this is different - an earthed conducting plate of quite nominal thickness is usually sufficient to do the job compared to that required to quench a magnetic field.


  Is there perfect equation.


Some would say all equations are perfect. However there are equations that give you skin depth but to calculate the attenuation effects in all but very simple cases realistically requires specialist software due to the number of interacting factors. A programme like quickfield will help and you can get a free student edition.