How to calculate Junction to Ambient temperature of LED?
Unlike other semiconductor devices such as transistors, attaching a heatsink to the device is not much use as it will block the light.

Page 12 of the datasheet states:


  The maximum forward current is determined by the thermal resistance between the LED junction and ambient. It is crucial for the end
  product to be designed in a manner that minimizes the thermal resistance from the solder point to ambient in order to optimize lamp life
  and optical characteristics. [Emphasis mine.]




Figure 1. The thermal resistance figures are only quoted for junction to solder point. This implies that junction to air (through the lens) is not significant.

You should be able to design to meet the solder pad specifications at the end of the document and follow the rating curves give.



Figure 2. Current safe operating area vs ambient temperature.

Further reading:


Cree's Long-Term Lumen Maintenance application note.
Cree's Thermal Management application note.