When it comes to the stone on a dancing diamond ring or any other diamond for that matter, high crown angles can cause light to bend at the wrong angles.
The ideal crown angle of a diamond is widely considered to be 34.5 degrees. Most of the big gem labs agree that slightly more or less, and the diamonds can still be seen as well-cut. 34.5 was devised to calculate the final exit route for the light ray reflected from within, which still allowed the observer to view the brilliance and dispersion.
The angle needs to be high enough for the light to go in and out from a range of directions, which go up in number when you add the upper girdle and star facets. The angles cannot be too steep, or the light reflected from the pavilion and moving towards the crown gets sent right back into the stone.
34.5 degrees ensures the light is dispersed to the maximum, while at 30 degrees, the observer would need to look at the stone parallelly.