Rosette formation: in which order are leafs created
Spiral Phyllotaxis and the Golden Angle

The disposition of the leaves in your picture is not as simple as it seems. That's a spiral phyllotaxis, which can be very complicated.

The plant in your image is a houseleek, a plant from the Genus Sempervivum.
In those plants, according to Jeremy Burgess (Introduction to plant cell developmnent):


  ... leaf primordia are separated from each other in an angular arc that approaches 137&deg;.


By the way, 137&deg; is (almost) the golden angle (&phi;).

Here is an image explaining the phyllotaxis of Sempervivum (from Burgess, 1989):



Even more interesting is the pattern made by the 137.5&deg; GA spiral of Aloe polyphylla:



Sometimes this can produce invisible spiral patterns, a phenomenon called parastichy:



Source: Burgess, J. (1989). An introduction to plant cell development. 1st ed. Cambridge [etc.]: Cambridge University Press.

PS: I know that this is off-topic, but I'd like to share this video from Cristóbal Vila, which deals with Fibonnaci sequence, golden ratio and golden angle in nature. Look at the flower disposition at 1:44 : https://www.youtube.com/watch?v=kkGeOWYOFoA