It's all about light and flow, but...
My opinion is that they grow a LOT faster when encrusting than when growing radially. If you want to maximize this growth you need to frag them fairly regularly, removing as much of the calcium base as you can without touching the living tissue. This will help them grow horizontally onto discs. If you look at frags that came from a farm you will see that a lot of people are growing them on tiles rather than discs now.
If you get them to grow on tiles you can frag them very easily.