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Peter Vizmuller's RF Design Guide , includes with the book some excell files for thermal resistance calculations. I tihink I have seen the complete ebook posted in the ebook section. If you want it and cannot find it I will upload it.
flatulent, I agree that you can use a spreadsheet, but not in complex situations, where you have tapered-fins, multi-angle fins, different manufacturing capabilities...etc.
Powerful software packages can do much more than just simple "cascaded" claculations.