maxwell's equations (which I never learned, still interested) are
   expanded for magnetic charges
   at [1]https://en.wikipedia.org/wiki/Magnetic_monopole#Maxwell's_equatio
   ns . i've noticed sometimes wikipedia's citations can give more
   information than the article, if helpful.
   it seems like those equations could possibly be used to plot a volume
   of magnetic charge as a function of field shape and strength.
   regarding mass of a magnet, it seems like the force it experiences on
   it is going to be proportional to its field strength. so you might want
   magnets where field strength / mass is greatest, and at the greatest
   distance from the magnet. unsure.

References

   1. https://en.wikipedia.org/wiki/Magnetic_monopole#Maxwell's_equations