The most important point in in using a magnet on a breast (or any other part) is getting the Mag Zone positioned and scaled correctly. Usually the centre of the Mag Zone needs to be about level with the front surface of the breast, and entered on it side-side. The Zone needs to be a bit larger that the breast as its influence falls off towards its edges (this can be controlled via the Falloff Graph). You will probably need to reduce the xScale of the zone somewhat, as breasts are longer than they are wide. If using two magnets at the same time, one on the right breast and one on the left breast, you should try to keep any overlap of the Zones to a minimum, as overlap can introduce asymmetry. Once you have positioned the Zones to your best guess, zTran the Mag to see what affect it has, if necessary adjust the Zones. Once the Zones are approximately right, you can start translating, and/or scaling the Mag. You will probably need to swap backwards and forwards between adjusting the Mag and the Zone a number of times. Use "Memorize" so you can can revert to a "Restore" point when a change does not have the desired effect.

Many people find magnets hard to use. Magnets are something you need to build up a feeling for over a period of time. There is no easy way to explain how to use them. In the end practise and experiment are the best tutors. Experiment on a simple shape, like a cube, or a flat surface like the GROUND plain to help you get a feel for it. Setting the Mag and Base to Outline Display Style can help you to have a clearer view. To adjust both breast at once you could try my "MirrorMags32" available at www.sharecg.com.