Since that's a simplification, it's not totally correct, but I think(and hope) that it's close enough to be correct for your experiment. The full equation is this:

You can see that the only thing that changed is the left hand side, and I replaced
B² / z with B * dB/dz. dB/dz is something from calculus.It's basically how fast B changes with respect to z. For example, if Bgoes from 5 T to 10 T in 5 meters, then dB/dz would be approximately 1T/m (Tesla per meter) over that distance. I say approximately because the magnetic field could change in a funny way, really fast at first,then slower later, so it could be different for different parts of z. But this would be the correct average value of dB/dz. And that's basically where the approximation comes in to play. In the simplified equation, I approximated dB/dz with B/z, which would roughly be the average value of dB/dz.

The simplified version should be enough if you want to find a rough value of the minimum magnetic field strength needed to levitate some Bismuth. If you were to try to compare the simplified version to the non-simplified equation, that could be tricky, because measuring dB/dzwould probably need a lot of measurements, and I think the tools to measure a magnetic field can be a bit expensive. However, it might be possible that wherever you buy your magnets from would have information on dB/dz (the rate of change of the magnetic field with respect to distance) for each magnetic. I'm not too sure about that, but it mightbe worth a shot. Good Luck!!