That equation is the condition necessary to cause the Bismuth to levitate - it's kind of a guide to what you will need in your experiment. If you want to calculate the diamagnetic force, there's an indirect but easy way to do that. The Bismuth will only have two forces acting on it - the diamagnetic force, and the force of gravity pulling down. The Bismuth levitates because the diamagnetic force is strong enough to counteract the force of gravity. In that case, the strength of the diamagnetic force will be the same as the force of gravity pulling down. If it was weaker, then the Bismuth would fall, and if it was stronger, then the Bismuth would shoot up into the air. So if we want to find the strength of the diamagnetic force, we can do so by finding the force of gravity pulling down on the levitating Bismuth, which would be its weight. Weight is mass * g (W = m * g), where g is a constant equal to 9.8 m/s² (meters per second squared), and m is in kg (kilograms). So if you get the mass of the Bismuth that is levitating in kilograms, and multiply it by 9.8, you'll get the weight of the Bismuth in Newtons. And as I said earlier, this has to equal the diamagnetic force acting on the Bismuth, or else it wouldn't levitate.