I know there is a very simple formula to calculate the power density of RF radiation at a certain distance from the source by knowing the transmitted power source. Which is:G/4Πr2 (where 4Πr2 is the surface area of a sphere and G is the antenna gain).

Is there any simple formula to calculate the elf magnetic field strength at a certain distance from the source? For example:if i measured an 45 milligauss magnetic field(ELF:50-60Hz)when the gauss meter is close attached to the measured object,how can i calculate the magnetic field strength at a distance of 10cm,20cm,1m etc.?

Thanks.