Force between two fixed magnets

Hi,

I am having trouble understanding the force calculation between two permanent magnet in COMSOL. My attached model can find the force but this force is different from the analytical expression for the force which can be found here: (Force between two cylindrical magnets) en.wikipedia.org/wiki/Force_be...Force_between_two_bar_magnets. In COMSOL the force will change if I change the air sphere size? Any idea about the correct size of sphere with respect to size of magnets?