We know that when we release a magnet in conducting pipe (aluminum, copper, etc..) it will be subjected to a magnetic force induced by the induced current. This magnetic field will be created in a manner opposite to the rate change of flux caused by the falling magnet. This in turn will slow down the magnet as you can see here.
I have seen on this forum message #6, the equation of the terminal velocity reached by the magnet during the fall where $v$ is the terminal velocity, $m$ is the mass of the magnet, $g$ is gravitational acceleration, $\rho$ is the resistivity of the tube, $R$ is the inner diameter (I am not sure why it is symbolized by R), $B$ is the magnetic flux density, $b$ is the length of the tube and $T$ is its thickness:
$$v = \frac{mg\rho}{4 \pi RB^2bT}$$
My questions are:
I have only seen the above equation of Physics Forums, I am not sure how it is derived? If this is difficult to answer, I would like to know if it has a name or if you have seen this equation before?
I want to perform this experiment and I would be capable of getting all the above factors except for the magnetic flux $B$. Do you have any idea how I can determine B experimentally?