How do I calculate the force exerted on a magnet by an electromagnet?

Hello. I am planning on creating some sort of milligram scale, but I ran into a problem with calculating a component of my scale. Say I have a small magnet which I know the mass of, and a electromagnet in the form of a solenoid in which I know the number of coils, the permeability of the core, and the length of the solenoid.

Now say I orient the electromagnet so that one of its poles are facing up, and I place the small magnet over the pole with guidance rails so that it doesn't flip. Is there an equation to calculate the height that the small magnet will levitate with respect to the current applied to the electromagnet? Would I require additional information, like the magnetic field strength (in teslas) of the small magnet? I am planning to use this method to calculate the weight of small masses.

I have started with B=uIN/L for the magnetic field strength of the electromagnet, but I cannot find a relationship for the force between an electromagnet and a normal magnet, nor have I found the equation relating B to force. Can someone help me on this one?

Oh, and as a side question: wasn't able2know once a forum? When did it change its layout to the current one?