As far as I can understand it you should use the "power" parameter and that will give you
a value in dBm (or dBm /10), so then you can map it to the CSQ value using the table in page 29.
We can also wait for an "official" answer on that from someone at SODAQ.
If you don't mind me asking, how are you getting the +NUESTATS power value and storing it?
Are you doing it with AT commands or are you using a library, and how?
Also, what board are you using? THe shield, the module or the arduino board?