This calculation would appear reasonable as a first-pass process for estimating a maximum permissible thermal resistance of a heatsink required to dissipate some total thermal load distributed across multiple sources, based on limiting worst-case junction temperature to some specified value. 0.0125°C/W (the calculated result) is in the same ballpark as 0.015°C/W, but on the wrong side of the fence by 20% from where one would prefer to be.
It depends on which “ambient” the heatsink in question is actually exposed to. If it’s sitting in front of the cool air intake that’s one thing, if it’s sitting at the exhaust and only getting air that’s been pre-heated by other co-located equipment, that’s quite another.
That math standing as it is, there are some other observations on the larger scenario that seem to warrant mention:
If one’s taking that number off the nameplate of the fans in question, it’s very likely to be inaccurate in practice. This post may help illuminate the reasons why. Insofar as heatsink thermal resistances tend to be based on some prescribed airflow condition, it’s a rather important detail.
As written, this would appear to imply that the the exhaust airflow is increasing in temperature by 15°C as a result of passing through the exhaust port. In practice, such results would likely suggest that the internal temperature probe is not well placed to capture the maximum internal air temperature.
Radiator…? Additional 10-15°C temperature? This causes me to question the degree of match between the idea in my mind and that which lies in yours. Pictures might be worth a thousand words, but one should take inflation into account…