Ballerina is not really a plant for division. Mine would always form one crown with many secondary crowns forming on it. To make a successful division, I would need more than one crown coming out of the ground with associated roots.

When I worked in a wholesale greenhouse, we would propagate the plant by cuttings from the secondary crowns forming on the large central crown. Again, the parent plants were not candidates for division since they only had one crown coming out of the soil. We would use rooting hormone on the cuttings. Success of the cuttings under greenhouse conditions was not high, only 50-60%. I always felt this was the reason Ballerina is not as widespread and available as it should be. It is a beautiful plant, very hardy.

I guess "ignorance is bliss". I had 3 Geranium 'Ballerina' plants that were about 3 years old. I dug up the largest one, cut the roots or pulled them apart and replanted the divisions directly into my garden. I did this in late spring or early summer - I don't remember exactly. All the divisions survived and the following season, these divisions were blooming and were almost the size of the original plant I had divided.

After that "experiment" proved to be successful, I divided one of the two remaining untouched plants the same way with the same results.

Here is a link that might be useful: Mountain View Experimental Gardens