Hi,
under "absolute maximum ratings"
there is
* 2W @ TA = 25°C and
* 30W @ TC = 25°C
--> If you keep on this, then you are on the safe side ... What "rule of thumb" do you expect?
I agree: the usual way is to calculate with junction temperature. But this is only "calculation", because usually you can´t measure junction temperature.
***********
The datasheet also gives the "Power derating chart" in Figure 4.
--> here you simply can see how much power you may dissipate at the given case temperature and vice versa.
It tells you the max. case temperaure of 150°C with "no power dissipation" --> thus (independent of any thermal resistance) the junction temperature is 150°C, too.
--> so you may expect TJ(max) to be 150°C.
The chart also tells you that you may dissipate 30W at 25°C case temperature.
Now assume the TJ(max) is 150°C... then you have a delta t of (150°C -25°C) at 30W.... which gives a R_th_JC of 4.2 K/W.
I think this is all you need to know.
Klaus